docs(backlog): split external coordination

[aatuh]

Backlog ticket

• Ticket ID: EXT-001
• One backlog ticket only; unrelated work was excluded.

Final commit

• PR title uses conventional commit syntax.
• Final squash commit body will include Refs: EXT-001.
• Breaking change: not applicable.

Verification

PASS GOWORK=off GOTOOLCHAIN=local make docs-check

Classification

• Compatibility: No public effect.
• Security: No new trust boundary or sensitive-data handling.
• Documentation/release notes: Backlog governance updated; release notes not applicable.
• Generated files: none.
• Benchmarks: not applicable.
• Migration: none.

This splits GitHub configuration, credentialed provider work, publication, independent review, live dependency triage, and elapsed-time proof into explicit EXT tickets. Local tickets may be [pr] only after their own source/checks/commit are complete; [x] still requires external evidence and protected merge proof.