fix(session): attribute MCP work to caller session

[agammann]

What / Why / How

Fixes #1198. When two Claude Code sessions share a project, MCP work can be attributed to the most recently started session instead of the caller.

Resolve the caller from the explicit CLAUDE_SESSION_ID override, then Claude Code's live sessions/<parent-pid>.json file, then the inherited CLAUDE_CODE_SESSION_ID. Keep the existing session DB fallback for other hosts. Apply this to indexed content, session events, persistent tool-call counts, restored stats, and the per-conversation ctx_stats lookup. Reading the live file on each call also handles /clear when the inherited ID is stale.

Affected platforms

• Claude Code

Test plan

• pnpm exec vitest run tests/session/caller-session.test.ts tests/session/event-emit.test.ts tests/session/server-tool-call-wiring.test.ts: 18 passed.
• pnpm run typecheck: passed. Build and bundle assertions passed.
• MCP stdio check with two seeded sessions: the built server inherited the neighbor's ID but read the caller's live session file; a ctx_execute tool-call row and sandbox event were both credited to the caller.
• Full pnpm test on Windows: 4,687 passed, 41 failed, 77 skipped. The failures were in shell/Python, symlink-permission, /tmp, and npm pack tests; the targeted session suites passed.
• Live Claude Code CLI session and transcript were not run because the CLI is unavailable in this environment.

Checklist

• Tests added/updated
• npm test passes
• npm run typecheck passes
• Docs updated if needed (internal attribution change; no docs updated)
• No Windows path regressions (uses path.join())
• Targets next branch (unless hotfix)