Harden Finn-loop for public use

[finna]

What changed

• Namespaced the commands as /finn-spec, /finn-build, and /finn-review.
• Made installation detect the target repo's default branch and Linear workflow state instead of assuming Henry's setup.
• Excluded blocked issues and human-escalated PRs from automated retries.
• Anchored reviews to the exact head SHA and required green GitHub checks plus clean mergeability before loop-approved.
• Added clean-worktree, one-builder, skill-reload, Linear–GitHub integration, and unattended-usage safeguards to the README.
• Added a zero-dependency validator and GitHub Actions check for the public skill contract.

Why

The first public version captured the factory's core idea, but several Henry-specific assumptions could make a fresh installation recycle blocked work, collide with Claude's /review command, or treat a PR as merge-ready before CI was green.

User impact

The copy-paste setup is now portable across repositories and Linear teams, while retaining the deliberately small three-skill model and the human merge gate.

Validation

• All three skill folders pass the skill validator.
• Simulated installation replaced every TEAM placeholder with ENG and preserved valid YAML.
• README links and eight hardening contracts were checked programmatically.
• git diff --check passed.
• Credential-pattern scan passed.
• GitHub Actions Validate passed on commit 12abdbe.

Follow-up after merge

Tag the merged commit as v0.1.0 before recording the public walkthrough.