Publish GitHub Code Context workflow and agent installation guide

[alphali]

Summary

Publish the existing self-contained runner as a public customer installation artifact, with an immutable-release checksum manifest and agent-executable setup instructions. Distinguish local plugin, provider billing, GitHub App approval, and Doable repository connection.

No runner behavior, API contract, database schema, or local plugin version changes. Existing customer copies do not auto-update. Backend deployment is not required for this publication.

Change Set: docs/change-sets/github-context-public-distribution-v1.yaml. Companion internal runbook update is on codex/github-context-public-release.

Verification

• npm test: four tests passed, including tampered artifact/metadata rejection.
• actionlint v1.7.7 passed for the template and CI workflow.
• Both embedded Python helpers compiled.
• Artifact is byte-identical to the reviewed generated source; SHA-256 is recorded in release.json.
• Existing local plugin package digest is unchanged.
• No customer secrets, production mutations, or paid runs performed.

Rollback

Restore the previous approved customer workflow copy. No deployment or migration required.