feat(evidence): record MCP computer use

[ElbertePlinio]

Summary

• record sanitized desktop, Android, and session mutation actions in one evidence run per session
• associate explicit screenshots with the active run without implicit capture
• finalize evidence through shared session destruction and dead-session reaping
• isolate evidence failures from MCP tool results and support evidence.enabled: false

Validation

• bun run typecheck
• bun run build
• bun run test — 75 files, 928 passed, 2 skipped
• bun run test:coverage — 75 files, 928 passed, 2 skipped; thresholds met
• two independent changed-HEAD local reviews clean

Refs #20