docs(phase-28): freeze checklist item sign-off scope

[xiwuqi]

Summary

This docs-only direct-to-main PR removes the Phase 28 execution blocker by freezing the formal Phase 28 contract on main.

It freezes:

• the Phase 28 formal name
• formal goals
• formal acceptance standards
• formal non-goals
• recommended monorepo impact range
• preconditions
• primary architecture risks
• must-be-deferred items
• recommended branch slug

This PR only changes repository-owned definition files:

• AGENTS.md
• docs/roadmap.md
• docs/architecture/phase-28-handoff.md

Why this PR exists

This is not a Phase 28 implementation PR.
Its only purpose is to converge the Phase 28 scope freeze into main so the next round can return to the Phase 28 execution template from latest main.

Merge of this PR is required before Phase 28 execution can begin.

Scope boundaries

This PR does not modify runtime, replay, observability, analytics, queue, worker, API, CLI, or web behavior.
It does not start checklist item sign-off implementation.
It does not expand into README updates because README is not a blocker for this scope freeze.

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