fix(mcp): resolve ctx_index relative paths from project dir

[Tomodad]

Summary

• Resolve ctx_index(path) relative file paths against context-mode's detected project directory instead of the MCP server process cwd.
• Preserve the original relative path as the source label when no explicit source is provided.
• Add a regression test that starts the MCP server with CONTEXT_MODE_PROJECT_DIR and indexes a relative file path via ctx_index.

Test plan

• pnpm run typecheck
• pnpm exec vitest run tests/core/server.test.ts --testNamePattern \"ctx_index file paths\" — blocked locally by missing better-sqlite3 native binding under Node 24 (Could not locate the bindings file).

🤖 Generated with Claude Code

[mksglu]

No need seperate file for that.

[Tomodad]

Addressed — removed the separate src/project-path.ts helper and kept the relative-path resolution logic local to src/server.ts. Also removed the helper-only tests so the change stays small.

Verified locally:

• pnpm run typecheck
• pnpm run build