T-130: MCP server over HTTP+SSE for /ide integration

Seventh slice of T-99. clide now advertises itself to Claude Code's
/ide command and serves a working MCP endpoint over HTTP+SSE per
D-73 (the Q-33 transport decision, locked in this commit).

What lands:
* D-73 — MCP transport for /ide is SSE over HTTP. Resolves Q-33;
  references D-68 + D-72.
* lib/src/ipc/mcp_server.dart — McpServer class. localhost HTTP
  listener on a random port; GET /sse opens a long-lived SSE stream
  with an initial endpoint event carrying the session id; POST
  /messages?sessionId=... accepts JSON-RPC requests and replies via
  the matching SSE stream. JSON-RPC handlers for initialize,
  tools/list, tools/call.
* Discovery file at $HOME/.claude/ide/<pid>.lock with the workspace
  + url so `/ide` can find us. Removed on stop.
* The two /ide minimum tools (mcp__ide__getDiagnostics,
  mcp__ide__executeCode) ship as stubs — real implementations need
  the analyzer integration / a clide eval surface, both follow-ups.
* main.dart starts the MCP server alongside the unix IPC server on
  daemonClientFactory and project switch. Failure non-fatal — the
  UI runs without MCP.
* 12 server tests cover lifecycle (start/stop, lock file), unknown
  paths, full JSON-RPC round-trip for all four methods, error
  responses, and edge cases (unknown session, malformed JSON,
  notification without id).

The "Claude Code's /ide discovers and connects" smoke is deferred to
T-131 wrap-up since it needs a real Claude Code session against the
running app — out of scope for unit/widget tests.

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -37,6 +37,12 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   to 16 recent matching events per subsystem (D-6), and streams new
   ones as JSON lines. C client loops on `data.streaming` ack. Slow /
   broken subscribers drop themselves without blocking the bus.
+- MCP server over HTTP+SSE (T-99 / T-130, per D-68 / D-73). Localhost
+  HTTP listener advertises via `$HOME/.claude/ide/<pid>.lock` so
+  Claude Code's `/ide` discovers it. JSON-RPC 2.0 with the two
+  minimum `/ide` tools shipped as stubs
+  (`mcp__ide__getDiagnostics`, `mcp__ide__executeCode`); real
+  implementations follow.
 - C `clide` shell client at `native/clide-cli/clide.c`. Walks CWD up
   to the git root, hashes to the per-workspace socket (D-70), ships
   argv. `make clide-cli` builds it; on PATH, `clide status` works