feat: add Composio Gmail connector vertical slice

[irresi]

Summary

• add an experimental, unofficial Composio Gmail → GBrain runtime
• verify signed V3 webhooks before durable SQLite admission
• rehydrate Gmail messages through the exact connected account and normalize metadata by default
• upsert deterministic pages through a source-bound GBrain MCP client
• add replay, crash recovery, lease fencing, source isolation, privacy, and synthetic restart E2E coverage
• add public security/threat-model docs, pinned CI actions, Dependabot, gitleaks, and branch-protection-compatible checks

State

• Implemented and synthetically verified: webhook verification, durable inbox, deterministic normalization, source-locked sink, retry, stale-lease fencing, restart recovery
• Implemented but not live-verified: Composio Gmail hydration and GBrain MCP delivery
• Not implemented: trigger creation, backfill/reconciliation, tombstones, Calendar, Airbyte
• Not deployed: no real account, trigger, webhook URL, OAuth scope, or GBrain source was changed by this PR

Verification

• bun run check — Biome, TypeScript, 45 tests, production Bun build
• bun audit — no vulnerabilities
• gitleaks git --staged --redact --no-banner — no leaks
• production bundle startup smoke: GET /healthz returned HTTP 200
• independent pre-commit review performed; prior HTTPS, unsafe Markdown, immutable routing, lease claim, and stale-worker fencing findings were fixed before submission

Next checkpoint

Operator approval is required before selecting a real Gmail connected account, changing/creating a Composio trigger, exposing a webhook, creating a source-bound GBrain credential, or running a live read-only canary.