Add autonomous build + deploy-autofix workflows

[EvieHwang]

Adds the wiring to let a merged spec build itself to a finished PR without a human in the loop — opt-in per feature, fenced, and with an explicit escalation channel.

What's here

• .github/workflows/autonomous-build.yml — fires only when a spec: PR is merged and carries the autonomous-build label. Runs /build headlessly on Opus 4.8 to a green suite and opens a build: PR. The label is the opt-in: unlabeled spec merges do nothing, so autonomy is never the default.
• .github/workflows/deploy-autofix.yml — on a failed deploy, hands a fresh agent flyctl + logs to diagnose and fix, opening a fix: deploy … PR (not a push to main). Enable after the build loop is proven; it's the riskiest piece and gets the tightest leash.
• AUTONOMY.md — the contract: how it runs, the escalation rules, the fences, and the prerequisites.

The design decisions, made literal

• Opt-in per feature via the autonomous-build label — small/confident work goes autonomous, novel/dangerous work stays in the loop.
• Escalation = notification. Async there's no session watching, so a blocked run opens a build-blocked: / deploy-blocked: issue and stops rather than spinning or faking green.
• Fences are permissions, not vibes. Read logs and push code, yes; set secrets or change access, never — those bounce back by design.

Before this can run (one-time, owner-only)

1. Install the Claude GitHub app and add ANTHROPIC_API_KEY to repo secrets (repo admin).
2. Confirm FLY_API_TOKEN is in repo secrets.
3. Create the autonomous-build label.
4. In deploy-autofix.yml, set workflows: ["…"] to your deploy workflow's name; uncomment the toolchain setup in autonomous-build.yml for your stack.

Out of scope

Auto-merge of any PR (every PR stays a human gate), and direct Fastmail notification (GitHub issues are the default channel; wiring Fastmail to the two labels is a later enhancement).

These are framework templates with project-specific values marked for fill-in. Review, then merge to land the pattern.