chat-app is half-migrated: the PR body is the spec, but planning/ is still a merge gate #70

What

chat-app's AGENTS.md states the PR-body-as-spec convention:

The spec for a change is its PR body, not a committed file.

and in the same file keeps the convention it replaces:

• deferred work goes to planning/deferred/, self-contained, with a revisit trigger
• just check-planning validates planning/deferred/ frontmatter
• just check-planning, just check-adrs and just check-links are merge gates

The convention is explicit that there are exactly two places for something that
outlives a PR: an ADR in docs/adr/, and a GitHub issue. planning/deferred/ is the
third state it says does not exist. chat-app is the only repo in the org still
running one.

How it got here

chat-app adopted the template shape (#6, "Adopt modern-di's lean planning
convention") without the routing. Its now-deleted PR template pointed at
planning/README.md#where-a-fact-goes and routed deferred work to
planning/deferred/, while modern-di's and faststream-outbox's copies routed to
docs/adr/ and GitHub Issues.

This is why "adopted the convention" was ambiguous across three repos: .github
adopted the routing while running the generic template, chat-app adopted the
template shape while keeping planning/, and only modern-di and
faststream-outbox had both.

The work

• Migrate the items in planning/deferred/ to GitHub Issues, self-contained enough to
  pick up cold. modern-di #439 did exactly this and is the reference.
• Delete planning/ and its validator.
• Drop just check-planning from the lint recipe and from CI.
• Update the AGENTS.md Workflow section so the deferred-work route reads "a GitHub
  issue", and remove check-planning from the commands section.

Keep

• just check-adrs. It validates docs/adr/ numbering, naming and revisit triggers,
  and is the only mechanical enforcement of the ADR revisit trigger anywhere in the
  org. It is unrelated to planning/.
• just check-links.

Note

chat-app's .github/PULL_REQUEST_TEMPLATE.md is being deleted separately (#53). That
deletion removes the file that stated the planning/deferred/ route, but not the AGENTS.md text or the CI gate, which is what this issue covers.

Revisit trigger

None; this is a half-finished migration to complete, not a decision to revisit.