fix(mcp): normalize root-ish path filters in codegraph_files (#426) (#466)

The agent (opencode/Gemini Flash on Windows) called codegraph_files with
path="/" and got "No files found matching the criteria.", which pushed it
straight back to Read/Glob. Indexed file paths are stored as
project-relative POSIX (e.g. "src/foo.py"), and the old startsWith filter
matched nothing for any of the root-ish or platform-flavored shapes an
agent might guess: "/", ".", "./", "", "\\", leading-slash and
leading-./ subpaths, or Windows backslash subpaths.

Normalize the filter (strip leading "/", "./", "\", bare "."; convert
"\" to "/"; trim trailing "/"), then match by exact equal or "<filter>/"
boundary — which also kills a sibling-prefix bleed where filter "src"
used to match "src-utils/...".

Validated on macOS + Linux (Docker) + Windows (Parallels) with 13 new
unit tests plus the existing mcp-input-limits/concurrent-locking
suites, and end-to-end through opencode in tmux (Big Pickle/OpenCode
Zen): codegraph_files [path=/] now returns the project tree and the
agent answers directly instead of falling back to Read.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>