docs(phase-27): freeze checklist item acknowledgment scope

[xiwuqi]

Summary

This PR removes the Phase 27 execution blocker by freezing a formal Phase 27 scope in repository-owned documentation.

It:

• names Phase 27 explicitly
• derives the next phase from existing deferred attestation-tracking and workflow-boundary items
• adds a formal Phase 27 handoff document
• updates the roadmap with a Phase 27 summary and handoff link
• updates AGENTS phase discipline to include the frozen Phase 27 name

Why this is needed

Before this PR, the repository did not contain a formal Phase 27 contract on main.
That meant the next phase could not start from main without relying on branch-local context or guesswork.

Scope boundaries

This PR does not start Phase 27 implementation.
It does not modify runtime, replay, observability, analytics, queue, worker, API, CLI, or web behavior.
It only adds the repository-owned contract needed to safely begin the next execution phase.

README is not a blocker for this scope freeze, so it is intentionally not changed in this PR.
README.md line 19 can be handled later as a separate minimal docs sync item.

Changed files

• AGENTS.md
• docs/roadmap.md
• docs/architecture/phase-27-handoff.md

Validation

• document structure self-check
• naming consistency self-check
• README sync decision self-check
• git diff --check

Note:

• there is no dedicated docs-only lint/check for these files in the current repo config
• pnpm exec biome check does not process these doc paths under the current config