feat(M4): YAML persona repository with hot reload

- Add YamlPersonaRepository: loads personas/*.yaml on startup; FileSystemWatcher hot-reload with 500ms debounce
- Re-introduce IPersonaRepository.PersonaChanged event (PersonaChangeKind: Added/Updated/Removed)
- Validation: required id/name/systemPrompt, BackendId.TryParse for backend, ToolRef.Parse for tools, duplicate-id rejected, malformed YAML throws InvalidOperationException
- DI fallback chain: YAML when personas dir exists -> InMemory in Development -> fail fast in non-Dev
- Seed personas/*.yaml for the 6 default DBA personas (orchestrator, explorer, analyst, performance, assessor, administrator)
- API csproj copies personas/*.yaml to output dir
- Tests: 13 new (101 total green); covers load, validation failures, duplicates, version stability, add/modify/delete events, watcher hot-reload
- README: new 'Personas (M4)' section, schema, hot-reload behaviour, how to add a persona

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: razeone

README.md

@@ -194,6 +194,38 @@ public class AgentPersona
 
 When the workflow engine invokes an agent, it uses the persona's `Backend` to look up the configuration and instantiate the appropriate `IChatClient`.
 
+## Personas (M4)
+
+Personas live as YAML files in a directory configured via `Personas:Directory` (defaults to `./personas` relative to the content root). Each file describes a single persona and is hot-reloaded when the file changes on disk.
+
+### Schema
+
+```yaml
+id: explorer                 # required, matches the persona's address
+name: Schema Explorer        # required, human-readable name
+backend: azure-openai        # required, one of: azure-openai, azure-foundry, openai, github-models, anthropic
+systemPrompt: |              # required
+  You discover database structure: schemas, tables, views, columns, indexes,
+  foreign keys. Use the SQL MCP tools to introspect; never modify data.
+tools: []                    # optional, list of mcp:<server>.<tool> refs
+guardrails:                  # optional
+  maxTokens: 2048
+  temperature: 0.1
+  topP: 0.95
+```
+
+### Hot reload
+
+The repository registers a `FileSystemWatcher` on the personas directory and debounces bursts of file events (~500 ms). On every reload it diffs the new snapshot against the previous one and raises `IPersonaRepository.PersonaChanged` for every add, update, or removal. Consumers can subscribe to the event to invalidate caches or re-prime workflows.
+
+If the directory is not present at startup the API falls back to the bundled `InMemoryPersonaRepository` in `Development` and fails fast in any other environment.
+
+### Adding a persona
+
+1. Create `personas/<id>.yaml` with the schema above.
+2. Save the file — the watcher reloads automatically; the new persona becomes available on the next `GET /v1/personas` call.
+3. Invalid YAML or a missing required field is logged and the previous snapshot is kept (the API does not crash).
+
 ## API surface (v1)
 
 | Method | Route                                  | Description                                              |
@@ -251,7 +283,7 @@ This avoids leaking SSE streams to anonymous clients while keeping the EventSour
   "Backends":        { /* per-backend connection settings */ },
   "Secrets":         { /* in-memory fallback secrets (development only) */ },
   "Mcp":             { "Servers": [ /* MCP servers */ ] },
-  "Personas":        { "Path": "./personas" },
+  "Personas":        { "Directory": "./personas", "Watch": true },
   "ConnectionStrings": { "Runs": "" }
 }
 ```
@@ -278,7 +310,7 @@ See the in-session plan for the full P0/P1/P2 backlog. Milestones:
 
 - **M2 (EF Core)**: ✅ In progress — SQL Server persistence and migrations have landed; integration tests & handler atomicity are in this wave.
 - **M3 (real LLM backends)**: ✅ Complete — Azure OpenAI, OpenAI, GitHub Models, and Anthropic adapters wired through `IChatClientFactory`. Azure Foundry deferred to M3.5.
-- **M4** (YAML personas + hot reload)
+- **M4 (YAML personas + hot reload)**: ✅ Complete — see the "Personas (M4)" section above.
 - **M5** (real workflow engine on `Microsoft.Agents.AI.Workflows`)
 - **M6** (MCP SQL server)
 - **M7** (MCP client wiring)

personas/administrator.yaml

@@ -0,0 +1,9 @@
+id: administrator
+name: Administrator (read-only)
+backend: azure-openai
+systemPrompt: |
+  You report on backups, users, roles, configuration, and high-availability
+  state. Do not perform destructive operations in this phase.
+tools: []
+guardrails:
+  temperature: 0.1

personas/analyst.yaml

@@ -0,0 +1,9 @@
+id: analyst
+name: Query Analyst
+backend: azure-openai
+systemPrompt: |
+  You analyze SQL queries: parse, explain plans, predicate selectivity, and
+  likely bottlenecks. Recommend rewrites; do not execute DDL.
+tools: []
+guardrails:
+  temperature: 0.2

personas/assessor.yaml

@@ -0,0 +1,9 @@
+id: assessor
+name: Best-Practices Assessor
+backend: azure-openai
+systemPrompt: |
+  You audit databases against security, compliance, and operational best
+  practices. Produce a prioritized findings list with severity and remediation.
+tools: []
+guardrails:
+  temperature: 0.2

personas/explorer.yaml

@@ -0,0 +1,9 @@
+id: explorer
+name: Schema Explorer
+backend: azure-openai
+systemPrompt: |
+  You discover database structure: schemas, tables, views, columns, indexes,
+  foreign keys. Use the SQL MCP tools to introspect; never modify data.
+tools: []
+guardrails:
+  temperature: 0.1

personas/orchestrator.yaml

@@ -0,0 +1,10 @@
+id: orchestrator
+name: Orchestrator
+backend: azure-openai
+systemPrompt: |
+  You route DBA requests to the correct sub-agent (Explorer, Analyst,
+  Performance, Assessor, Administrator). Decide based on intent and produce a
+  short handoff message.
+tools: []
+guardrails:
+  temperature: 0.2

personas/performance.yaml

@@ -0,0 +1,9 @@
+id: performance
+name: Performance Engineer
+backend: azure-openai
+systemPrompt: |
+  You diagnose performance: missing/duplicate indexes, wait stats, blocking,
+  parameter sniffing. Propose targeted, reversible changes.
+tools: []
+guardrails:
+  temperature: 0.2

src/CloudEngAgent.Api/CloudEngAgent.Api.csproj

@@ -19,4 +19,8 @@
     <PackageReference Include="OpenTelemetry.Instrumentation.Http" />
     <PackageReference Include="OpenTelemetry.Instrumentation.EntityFrameworkCore" />
   </ItemGroup>
+
+  <ItemGroup>
+    <None Include="..\..\personas\*.yaml" Link="personas\%(Filename)%(Extension)" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
 </Project>

src/CloudEngAgent.Application/Abstractions/IPersonaRepository.cs

@@ -7,4 +7,23 @@ public interface IPersonaRepository
     Task<AgentPersona?> GetAsync(string id, CancellationToken cancellationToken);
 
     IAsyncEnumerable<AgentPersona> ListAsync(CancellationToken cancellationToken);
+
+    /// <summary>
+    /// Raised when the underlying persona source detects an add, update, or removal.
+    /// Implementations that cannot detect changes (e.g., in-memory) never raise this event.
+    /// </summary>
+    event EventHandler<PersonaChangedEventArgs>? PersonaChanged;
+}
+
+public enum PersonaChangeKind
+{
+    Added,
+    Updated,
+    Removed,
+}
+
+public sealed class PersonaChangedEventArgs(string personaId, PersonaChangeKind kind) : EventArgs
+{
+    public string PersonaId { get; } = personaId;
+    public PersonaChangeKind Kind { get; } = kind;
 }

src/CloudEngAgent.Infrastructure/Personas/InMemoryPersonaRepository.cs

@@ -20,6 +20,10 @@ public InMemoryPersonaRepository()
         _personas = SeedPersonas().ToDictionary(p => p.Id, StringComparer.Ordinal);
     }
 
+#pragma warning disable CS0067
+    public event EventHandler<PersonaChangedEventArgs>? PersonaChanged;
+#pragma warning restore CS0067
+
     public Task<AgentPersona?> GetAsync(string id, CancellationToken cancellationToken)
     {
         ArgumentException.ThrowIfNullOrWhiteSpace(id);