chore: migrate off the planning/ convention

[lesnik512]

Migrates modern-di-celery off the planning/ convention and onto PR-body-as-spec, with CONTEXT.md owning the vocabulary and docs/adr/ holding rejected alternatives. Part of modern-python/.github#67.

Replaces the two-axis planning/ + architecture/ convention with PR-body-as-spec,
CONTEXT.md for vocabulary, and docs/adr/ for rejected alternatives, per
modern-python/.github#67.

• planning/ deleted in full. changes/ is what git history already records; the six
  releases/*.md files were verified byte-identical to the published GitHub Release
  bodies (checked 3.0.2 and 2.1.0 with gh release view; the only difference is a
  trailing newline), so the Releases are the record. decisions/ held nothing but a
  .gitkeep, and there is no deferred.md — no work was left unscheduled anywhere in
  the change files, so no draft issues were needed.
• architecture/ carried one capability page. Its decisions were rescued into four
  ADRs, each keeping a revisit trigger: 0001 no connection provider for a task
  invocation (from the celery-di-integration design), 0002 no functools.wraps on
  the inject wrapper, 0003 rejecting *args/**kwargs alongside FromDI (with the two
  alternatives that were considered and refused), and 0004 connecting both worker
  signal pairs (from the open-root-all-pools change and the 3.0.1 release notes).
  The rest of the page was prose about mechanism that main.py already states.
• Two claims became INVARIANT tests, both verified by breaking them. The public
  surface is exactly the five documented symbols: it fails on an unprefixed public
  binding in init and on a helper promoted into all, and does not fail when
  internals are reshuffled behind private names or module imports. A FromDI
  parameter is absent from the signature Celery binds against: deleting the
  signature assignment, or swapping the hand-copied dunders for
  functools.update_wrapper, both turn it red, while narrowing the visible signature
  (dropping its return annotation) or narrowing what inject accepts leaves it green.
  That first break is the reason the test exists — the whole suite passed with the
  signature rewrite deleted, because the wrapper re-binds arguments itself and only
  the caller-side arity check moves. The other enforceable claims already had
  tests: dropping weak=False fails three lifespan tests, and dropping DITask's
  header reset fails test_ditask_resolves_without_inject.
• CONTEXT.md defines the three terms local to this package; Container, Provider,
  Group, Scope, Resolution and Override stay modern-di's. The glossary audit forced
  two source edits: README.md called the root container the "APP-scope container"
  twice, and main.py's comment called the visible signature the "stripped
  signature" — both brought to the spelling the rest of the repo already uses.
  Neither is a user-visible runtime string. A third avoid-entry ("request
  container") was cut: it appeared nowhere in the repo, so there was no synonym to
  reject.
• AGENTS.md gains Workflow and Where-a-fact-goes and loses the planning lanes.
  docs/agents/domain.md was deliberately not authored: this repo has no docs/
  directory before this change and no Agent skills section to link one from, so
  writing it alone would be a half-rollout of a separate convention.
• justfile and lint-ci drop index/check-planning/check-links; the offline lychee
  gate in _checks.yml replaces links.py (Add the offline link gate to the 18 green repos, and unpublish the ADRs in the two that have them .github#66) and reports
  0 errors over every .md.

Beyond the recipe, and the reason this is chore: and not docs: release.yml read
planning/releases/${GITHUB_REF_NAME}.md both as a mandatory gate for stable tags
and as the Release body source, so deleting planning/ would have failed every
future stable release. It is now modern-di/.github/workflows/release.yml verbatim
(only the PyPI project name differs): GitHub's generated notes, prerelease flag
only. This retires the mandatory-curated-notes policy, following modern-di's
1ea74ee (modern-python/modern-di#449); a release wanting prose is edited after the
fact with gh release edit <tag> --notes-file.

Verified: just lint-ci clean, just test-ci 16 passed at 100% coverage, lychee
--offline 0 errors, and no remaining reference to planning/, architecture/,
check-planning, check-links or convention-version anywhere in the repo.

Verification

• just lint-ci — clean (eof-fixer, ruff format, ruff check, ty).
• Test suite green at the repo's 100% coverage gate.
• Offline link gate (lychee --offline --no-progress '**/*.md') — 0 errors.
• No planning/, architecture/, check-planning, check-links or convention-version reference remains.

Note

release.yml previously read planning/releases/<tag>.md, both as a hard gate for stable tags and as the Release body source; deleting planning/ without changing it would have broken the next stable release. It now uses GitHub's generated notes, matching modern-di post-modern-python/modern-di#449. This retires the mandatory-curated-notes policy.