feat: MCP server — expose session data to Claude Code (v0.4.0)

Adds `claudewatch mcp` subcommand implementing a JSON-RPC 2.0 stdio
server compatible with the Model Context Protocol. Exposes three tools:
get_session_stats, get_cost_budget, get_recent_sessions.

Configure in ~/.claude.json mcpServers to make tools available inside
Claude Code sessions.

Author: blackwell-systems

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 All notable changes to claudewatch are documented here.
 
+## [v0.4.0] - 2026-02-28
+
+### Added
+
+- **MCP server** — new `mcp` subcommand runs a JSON-RPC 2.0 stdio server compatible with the [Model Context Protocol](https://modelcontextprotocol.io). Exposes three tools to Claude Code and other MCP clients: `get_session_stats` (most recent completed session with cost and token breakdown), `get_cost_budget` (today's estimated spend vs a configurable daily budget), and `get_recent_sessions` (last N sessions with friction scores and cost, default 5, max 50). Start with `claudewatch mcp` or add `--budget <USD>` to enable budget tracking. Configure in `~/.claude.json` under `mcpServers` to make the tools available inside Claude Code sessions.
+
 ## [v0.3.0] - 2026-02-28
 
 ### Added

README.md

@@ -207,6 +207,7 @@ claudewatch track --compare
 | `track` | Snapshot metrics to SQLite, diff against previous |
 | `log` | Inject custom metrics (scale, boolean, counter, duration) |
 | `watch` | Background daemon with desktop alerts on friction spikes |
+| `mcp` | Run an MCP stdio server for Claude Code integration |
 
 ### `claudewatch fix`
 
@@ -235,6 +236,36 @@ claudewatch watch --stop              # stop background daemon
 
 Notifies on: friction spikes, new stale patterns, agent kill rate increases, zero-commit streaks.
 
+### `claudewatch mcp`
+
+Run claudewatch as an MCP ([Model Context Protocol](https://modelcontextprotocol.io)) stdio server. This makes your session data directly queryable from inside Claude Code sessions without leaving the terminal.
+
+```bash
+claudewatch mcp                    # start MCP server on stdio
+claudewatch mcp --budget 20        # enable daily budget tracking ($20 limit)
+```
+
+**Configure in Claude Code** by adding to `~/.claude.json`:
+
+```json
+{
+  "mcpServers": {
+    "claudewatch": {
+      "command": "/usr/local/bin/claudewatch",
+      "args": ["mcp", "--budget", "20"]
+    }
+  }
+}
+```
+
+**Tools exposed:**
+
+| Tool | Description |
+|------|-------------|
+| `get_session_stats` | Most recent completed session: cost, tokens, duration, project |
+| `get_cost_budget` | Today's estimated spend vs your daily budget |
+| `get_recent_sessions` | Last N sessions (default 5, max 50) with friction scores and cost |
+
 ### `claudewatch metrics --json`
 
 Machine-readable JSON export for all metrics sections. Use for time-series analysis, cost dashboards, CI/CD integration, or custom queries.