Support explicit subagent session files

[wenerme]

Summary

• Add session_file to custom agent frontmatter, the Agent tool parameters, scheduled subagents, and spawn/run configuration.
• Resolve relative session_file paths from the requested agent cwd before worktree isolation, so stable project session lanes do not get written into temporary worktrees.
• Open explicit session files through SessionManager.open(...) for append/resume, create missing parent directories, and treat session_file as implying persistence.
• Document the feature and add focused filesystem coverage.

Closes #131.

Validation

• ./node_modules/.bin/biome check src/ test/
• npm run typecheck
• npx vitest run test/agent-runner.test.ts test/agent-manager.test.ts test/custom-agents.test.ts test/invocation-config.test.ts test/session-file.test.ts
• npm run test
• git diff --check

Note: npm run lint returned ESLint output (JSON parse failed: EOF...) in this local harness, with no Biome diagnostics. The script's underlying command, biome check src/ test/, passed directly.