Proposal: Rewrite synapse-sentinel org README.md with ecosystem vision + repo links

[lexi-chief-of-staff]

Current State

synapse-sentinel org README.md is missing (.github/README.md not found)—org page defaults to repo list without vision. Gate README covers quickstart but lags on PR#73 (v2), CLI commands, 16 open issues. [from: RepoStatus repo=synapse-sentinel/gate]

Proposed Structure

1. Vision: Synapse Sentinel—AI ecosystem for code/health/life ops. Citizens (Lexi/Forge) powered by gate (auth), bifrost (infra), core agents.

2. Quickstart: git clone synapse-sentinel/gate, Podman setup, Lexi Bot App install.

3. Repos:

   Repo	Purpose	Status
   gate	GitHub App auth/CLI	10 PRs open 🚧
   bifrost	Webhook/DB/Redis	Green ✅
   lexi-agent	Lexi CoS	Stable
   prefrontal-cortex	Planning	Planning

4. Contribute: Label PRs needs-review, run php artisan lexi:doctor.

5. Roadmap: Phase 2 business logic post-v2 gate merge.

Full markdown attached below—ready for PR.