chore: migrate off the planning/ convention

[lesnik512]

Migrates modern-di-fastapi 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/ held one design record whose content is the
  diff it describes, and git history is that record; releases/ held six notes
  verified byte-identical to the published GitHub Release bodies (3.0.1 and
  2.10.0 diffed against gh release view, differing only by a trailing newline
  GitHub adds), and the Releases are the record. deferred.md was empty and the
  change file listed non-goals rather than unscheduled work, so no issues needed
  drafting and there is no DRAFT-ISSUES file.
• planning/decisions/ was empty, so ADR-0001 is rescued instead from the
  rationale buried in architecture/container-lifecycle.md and _compose_lifespan:
  why this package owns its lifespan composition rather than calling FastAPI's
  private _merge_lifespan_context. It keeps a revisit trigger.
• architecture/ carried three prose pages about mechanism, all readable from
  main.py. The one enforceable claim was glossary.md's "the providers are the
  single source — there is no separate hardcoded table", now the INVARIANT test
  tests/test_connection_kinds.py. Verified against three cases: replacing
  classify_connection with an isinstance ladder fails it, naming the two
  providers literally in setup_di fails it, and narrowing the dispatch to
  HTTPConnection-typed providers — a change that restricts rather than
  duplicates — leaves it green.
• CONTEXT.md defines only the two terms local to this integration; Container,
  Provider, Group, Scope, Resolution, Override and Connection stay modern-di's
  and are not redefined here. Both Avoid entries were audited against real
  usage and both had live stragglers, so the stragglers were fixed rather than
  the entries dropped: "per-request"/"request container" in README.md and in
  main.py's Dependency.call parameter, now per-connection; and "appends a
  lifespan ... merges with any existing lifespan=" in README.md's API table, now
  composes/nesting. Both README edits are user-visible strings — that table is
  rendered on PyPI. tests/test_routes.py's build_di_container parameter was
  renamed to connection_container for the same reason. test_websockets.py keeps
  request_container for the REQUEST-scoped child it builds by hand: that is a
  scope name for a real child container, not the per-connection one.
• AGENTS.md gains Workflow and Where-a-fact-goes, loses the planning lanes and
  the architecture/ promotion rule, and states the ty-suppression spelling the
  justfile does not. docs/agents/domain.md is deliberately absent: this repo has
  no docs/agents/ tree and AGENTS.md has no Agent skills section to link one
  from, so authoring domain.md 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), reporting 0
  errors over 32 unique links.

Beyond the recipe, and the reason this is chore: and not docs: release.yml
sourced the Release body from planning/releases/.md and hard-required one
for every stable tag, so deleting planning/ would have failed every future
stable release. It now uses GitHub's generated notes, byte-identical to
modern-di's post-migration workflow, which dropped the same gate in 1ea74ee
(modern-python/modern-di#449). This retires the mandatory-curated-notes policy;
a release wanting prose is edited after the fact with gh release edit.

Verification: just lint-ci clean, just test-ci 9 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
tree.

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.