Spec 038 — rule_fired trace event for Tier-3 rule fires

[codemonkeychris]

Summary

• Adds a structured {kind: \"rule_fired\", rule, code, confidence, evidence, file, line, mode} row to mur check --trace whenever a Tier-3 rule attaches a suggestion to a diagnostic.
• Closes the EC3-final watch-item: per-rule firing-rate audits collapse from multi-step content scans against events.jsonl agent tool outputs to a 1-line jq '.kind==\"rule_fired\"' over the trace file.
• Tier-2 hits deliberately don't emit this row — Tier-2 firing rate is visible via the opt-in MUR_TELEMETRY=1 channel; the trace event exists specifically to make Tier-3 firings discoverable without telemetry opt-in.

Changes

Code (3 files):

• Suggestion record gains bool IsRule = false (positional default; existing call sites unchanged).
• SuggesterOrchestrator sets IsRule: true only at the rule-winning return site.
• TraceWriter.WriteRuleFired(...) with same 1024-char evidence truncation + path sanitization as diag rows.
• CheckCommand.EmitDiagnostics invokes it on rule-flagged suggestions.

Tests (5 new):

• TraceWriterTests: row-schema, evidence-truncation under 2 KB, file-path sanitization for <external> paths.
• CheckCommandPipelineTests: end-to-end emit-on-rule + no-emit-on-tier2.
• 227/227 in the CheckCommand test area pass.

Docs:

• CHANGELOG entry under Spec 038 — mur check did-you-mean.
• Spec/tasks watch-item marked LANDED with implementation summary.
• docs/reference/mur-check-did-you-mean.md trace-schema section expanded to enumerate all four row kinds (diagnostic, command, rule_self_disabled, rule_fired).

Follow-up

Remaining EC3-final watch-items + Phase-4 cleanup work tracked in #252 (filed alongside this PR).

Test plan

• dotnet build src/Reactor.Cli succeeds (.NET 10, warnings-as-errors clean).
• CheckCommand tests pass (227/227, x64).
• Reviewer spot-checks the new trace row shape against the existing rule_self_disabled schema for consistency.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds a new rule_fired JSONL trace row to mur check --trace so Tier-3 rule firings are directly discoverable from trace output (without requiring telemetry opt-in), and updates tests/docs to lock in the schema and behavior.

Changes:

• Add Suggestion.IsRule (default false) and set it only when a Tier-3 rule wins in SuggesterOrchestrator.
• Add TraceWriter.WriteRuleFired(...) and emit kind: "rule_fired" rows from CheckCommand.EmitDiagnostics when IsRule suggestions are attached.
• Add unit + pipeline tests and update docs/CHANGELOG to describe the new trace row kind.

Show a summary per file

File	Description
tests/Reactor.Tests/CheckCommandTests/TraceWriterTests.cs	Adds schema/truncation/path-sanitization unit tests for the new rule_fired trace row.
tests/Reactor.Tests/CheckCommandTests/CheckCommandPipelineTests.cs	Adds end-to-end tests verifying rule_fired emission for Tier-3 and non-emission for Tier-2.
src/Reactor.Cli/Check/TraceWriter.cs	Implements WriteRuleFired + RuleFiredRow with evidence truncation and path sanitization.
src/Reactor.Cli/Check/SuggesterOrchestrator.cs	Extends Suggestion with IsRule and marks only rule-winning suggestions as IsRule: true.
src/Reactor.Cli/Check/CheckCommand.cs	Emits rule_fired trace rows when an IsRule suggestion is attached to a diagnostic.
docs/specs/tasks/038-mur-check-did-you-mean-implementation.md	Marks the watch-item as landed and documents the implementation + tests.
docs/reference/mur-check-did-you-mean.md	Updates trace docs to enumerate rule_fired alongside other row kinds.
CHANGELOG.md	Adds a changelog entry describing the new rule_fired trace event.

Copilot's findings

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• Files reviewed: 8/8 changed files
• Comments generated: 2

[codemonkeychris]

Addressed both Copilot CR comments in 739403e:

1. JsonDocument disposal in pipeline test — refactored Emit_writes_rule_fired_trace_event_when_rule_suggestion_attached to parse each line inside a using block and detach the matching row via JsonElement.Clone() so assertions outlive the parsed document without leaking pooled buffers.
2. jq one-liner — '.kind=="rule_fired"' evaluates the expression per row (emits a boolean stream); correct filter is 'select(.kind=="rule_fired")'. Fixed in the spec/tasks watch-item entry, the TraceWriter test comment, and issue Spec 038 — Phase 4 cleanup: targeted-prompt batch, guardrail retrofit, Checkpoint D, ranker training #252's §A.4 (same example had propagated).

27/27 trace + pipeline tests still pass.

[codemonkeychris]

CI failure on Integration Tests is unrelated flake — TemplatePackageTestFixture ctor timed out at 3:42 wall on dotnet new install --debug:custom-hive (budget 120 s). Diagnosis:

• Failure is in tests/Reactor.IntegrationTests/Packaging/CreateTemplateTests.cs:268 — template-engine install on a cold CI runner. Nothing in 739403e touches packaging.
• Prior commit b2e6c8e in this PR had all 7 checks green including Integration Tests.
• Recent main CI is 8/8 green.
• dotnet new install cache thrash is the known flaky surface called out in spec 038 EC3-original; the recent template-typo fix (Spec 038 Phase 3 — Class-A rules, correctness fixes, EC3 results, template typo fix #250) addressed the duplicate-identity failure mode, but the underlying cold-runner install can still stall.

Re-triggered the failed job via gh run rerun --failed.