Configure issue intake labels and publish product changelog

[JRH89]

Changes

• Issue Chat can explicitly add the saved intake label when publishing a reviewed issue; default off.
• Administrator-controlled per-repository label bypass; default label gate retained by V46 migration, with confirmation and provider-charge warning. Assignee, body, open-state, budget and verification rules remain independent.
• Keep onboarding, public docs, user guide, README and readiness evidence coherent; do not claim remaining operational gates are complete.
• Prerendered milestone changelog with footer link, SEO metadata, sitemap and release/PR references.

Verification

93 frontend tests, five responsive layout tests, lint/typecheck/build and SEO checks passed. Full backend verification and targeted webhook/intake/publication tests passed. No live model calls or issue creations performed.

Closes #125