chore: migrate off the planning/ convention

[lesnik512]

Migrates modern-di-aiohttp 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. The four change files hold nothing git history does
  not; all six release notes are the published GitHub Release bodies verbatim
  (verified against gh release view for 2.0.0, 3.1.0 and 3.1.1 — identical but
  for GitHub's trailing newline), so the Releases are the record. deferred.md
  was empty and no change file carries unscheduled work, so no issues needed
  spawning.
• decisions/ was empty, but two rejected alternatives were buried in the design
  change file and in architecture/container-lifecycle.md, and both would be
  re-litigated by anyone who read only the code. They become docs/adr/0001 (two
  connection providers, one reference-only: dishka's single SESSION binding with
  contiguous scopes rejected) and docs/adr/0002 (scope from the can_prepare
  handshake probe: dishka's header comparison and route-table dispatch
  rejected). Each keeps a revisit trigger.
• architecture/ held prose about mechanism the module already states. The one
  enforceable claim was that exactly one connection provider carries a bound
  type, now the INVARIANT test in tests/test_connection_providers.py. Verified by
  breaking it: dropping bound_type=None from aiohttp_websocket_provider turns it
  red, restoring turns it green, and adding a third reference-only connection
  provider — a change that keeps the invariant — leaves it green.
• CONTEXT.md defines only the three terms local to this integration; the rest are
  modern-di's. The glossary audit cut the FromDI-marker entry (its only rejected
  synonym, "Depends", appears nowhere) and four of five Avoid synonyms: "app
  container", "global container" and "sub-container" have no use in the repo.
  "request container" is kept and is the one that earns its place — the public
  fetch_request_container() returns a SESSION-scoped container on a WebSocket, so
  the function name misnames the scope. No source edit was forced: no prose in
  the package, tests or README uses a rejected synonym today.
• AGENTS.md gains Workflow and Where-a-fact-goes, loses the lanes and the
  capability-page promotion rule, and sheds the Build & checks list the justfile
  already owned.
• justfile and lint-ci drop index/check-planning/check-links; the offline lychee
  gate 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), reporting 0 errors over 32
  unique links.

Beyond the recipe, and the reason this is chore: and not docs: release.yml read
planning/releases/.md as the GitHub Release body, so deleting planning/
would have silently changed every future Release. It now uses GitHub's generated
notes. The file is modern-di's post-migration release.yml verbatim, down to the
PyPI project name in one comment; that workflow dropped the same convention in
1ea74ee ("drop the planning/ directory and the curated-release-notes
convention", modern-python/modern-di#449), one of the five migration PRs #67
cites. This repo never carried the mandatory "Require curated release notes"
gate some siblings did, so nothing here becomes newly ungated — curated notes
simply stop being a source; a release wanting prose is edited after the fact
with gh release edit <tag> --notes-file.

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.