v1.9.0

Added

• MCP server: commitbrief mcp (ADR-0028). A new opt-in subcommand runs a Model Context Protocol server over stdio (JSON-RPC 2.0) so an AI agent or host — Claude Desktop, an agent runtime, an MCP-aware IDE — can invoke CommitBrief as a tool. The typical use is a self-review gate the agent runs before it submits code. It is stdlib-only (encoding/json + bufio line framing; no MCP SDK, no new dependency) and implements initialize, tools/list, tools/call, and ping.
• It exposes one tool, review, that runs the same review pipeline as commitbrief --json — diff acquisition, filtering, the pre-send guard and secret scanner, the cost preflight, the cache, the flaky-test pre-pass, and signal control — and returns the structured findings (JSON schema v1) plus a short text summary. The tool arguments expose the meaningful knobs (staged / unstaged / diff / provider / model / fail_on / min_severity / no_flaky).
• A fail_on gate is reported in the summary (the findings are still returned); a genuine failure — no repo or changes, a provider error, an aborted guard — comes back as an MCP tool error. The path reuses runReview rather than re-implementing the review, so there is zero behavioral drift from a terminal review. Fully additive — existing commands are unchanged.