feat(pm-stack): dispatch service — launch/track OpenCode sessions from GitHub Actions #2648

New pm-stack service pm-dispatch (Drew's request, 2026-10-30): an HTTP service on the laptop with two authenticated endpoints.

• POST /v1/sessions — launch an OpenCode session on the laptop immediately: creates a GitHub issue (optional, default on), enqueues a repo-health queue task, launches the session with the prompt, and marks the task claimed immediately (worker dispatch-opencode, session id in progress) so no lane double-books it.
• POST /v1/tasks — enqueue-only: puts a repo-task/v1 packet into the Supabase queue (repo-health-*-pilot), optionally with a GitHub issue.
• Auth: GitHub Actions OIDC (aud pm-dispatch, token's repository claim must equal the request's repo) or a static bearer secret.
• GET /health unauthenticated, aggregator-style build version.

Deployed on the laptop like the aggregator (install.sh + user systemd unit, loopback + tailnet bind). Public reachability (a Cloudflare tunnel hostname on amd-6-pm-stack) is a separate later step.