docs(changelog): document the MCP cold-start handshake fix

Adds the [Unreleased] note for the serve --mcp cold-start speedup: the server now answers an agent's opening handshake the instant it launches, so a fresh session's first tool call no longer races a still-warming-up server and falls back to grep/read.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -21,6 +21,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 ### Fixes
 
 - `codegraph_trace` now resolves an overloaded symbol name to its real implementation instead of an empty protocol/delegate stub. Tracing a flow through a heavily-overloaded API (common in Swift, Java, C#, and Go) could land on an unrelated no-op method that happened to share the name and report "no path"; it now picks the substantive definition the flow actually runs through.
+- CodeGraph's MCP server now answers an agent's opening handshake the instant it launches instead of blocking while the index loads, so a fresh session's very first tool call no longer occasionally races a server that's still warming up and falls back to grep/read. The first question in a new session now reliably goes through CodeGraph.
 - Indexing a project that contains only config-style files (YAML, Twig, or `.properties`) no longer misleadingly reports "No files found to index" — these files are tracked at the file level and are now counted as indexed. Thanks @luojiyin1987 (#357).
 
 ## [0.9.7] - 2026-05-28