docs(phase-29): freeze checklist item exception scope

[xiwuqi]

Summary

This PR removes the Phase 29 execution blocker by freezing a formal Phase 29 scope in repository-owned documentation.

It:

• names Phase 29 explicitly
• freezes the formal goals, acceptance standards, non-goals, monorepo impact range, preconditions, primary architecture risks, must-be-deferred items, and recommended branch slug
• adds a formal Phase 29 handoff document
• updates the roadmap with a Phase 29 summary and handoff link
• updates AGENTS phase discipline to include the frozen Phase 29 name

Why this is needed

Before this PR, the repository did not contain a formal Phase 29 contract on main.
That meant the next phase could not start from main without relying on branch-local context or guesswork.

Scope boundaries

This is a docs-only scope-freeze PR.
Its purpose is only to converge the formal Phase 29 definition into main.
It does not start Phase 29 implementation.
It does not modify runtime, replay, observability, analytics, queue, worker, API, CLI, or web behavior.
Merge of this PR is the gate that allows the next turn to re-enter the Phase 29 execution template.

README is not a blocker for this scope freeze, so it is intentionally not changed in this PR.

Changed files

• AGENTS.md
• docs/roadmap.md
• docs/architecture/phase-29-handoff.md

Validation

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

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