docs(phase-26): freeze checklist item attestation scope

[xiwuqi]

Summary

This PR removes the Phase 26 execution blocker by freezing a formal Phase 26 scope in repository-owned documentation.

It freezes:

• the formal Phase 26 name
• formal goals
• formal acceptance standards
• formal non-goals
• recommended monorepo impact range
• preconditions
• primary architecture risks
• must-be-deferred items
• the recommended execution branch slug

Why this is needed

The repository did not contain a formal Phase 26 contract.
That meant the next phase could not start without relying on chat context or guesswork.

This PR exists only to converge the Phase 26 contract into main.
It is not a Phase 26 implementation PR.

Scope boundaries

This is a docs-only scope-freeze PR.
It only changes:

• AGENTS.md
• docs/roadmap.md
• docs/architecture/phase-26-handoff.md

It does not start Phase 26 implementation.
It does not modify runtime, replay, observability, analytics, queue, worker, API, CLI, or web behavior.
It does not add checklist item attestation functionality.

README is not a blocker for this PR, so this scope freeze does not expand into README changes.

Frozen Phase 26 direction

Phase 26 is frozen as:

• Cross-Run Audit Catalog Checklist Item Attestations and Attestation Notes
• short label: Checklist Item Attestations

The handoff keeps the next phase intentionally narrow and derived from existing deferred evidence-layer boundaries.
It does not expand into payload persistence, copied binary artifact persistence, attachment-upload products, artifact-vault behavior, threaded collaboration, broader checklist orchestration, broader review workflow engines, fine-grained RBAC, multi-tenant access, dashboards, search, analytics, or any Phase 27 work.

Validation

• document structure self-check: passed
• naming consistency self-check: passed
• README scope 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

After merge

Only after this PR is merged into main may the repository return to the Phase 26 execution template.
Until then, Phase 26 implementation must not start.