chore: migrate off the planning/ convention

[lesnik512]

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.

What moves, and what is genuinely deleted

• planning/ deleted in full. changes/ and releases/ held nothing git
  history and GitHub Releases do not: the release notes were byte-identical to
  the published Release bodies. deferred.md was empty, so no issues needed
  spawning.
• The one decision record becomes docs/adr/0001, keeping its revisit
  trigger.
• architecture/ carried no capability pages. The only claim worth rescuing
  was the two-symbol public surface, now an INVARIANT test checked against three
  cases: __all__ growing and an unprefixed public binding both fail it, and
  narrowing the surface does not.
• CONTEXT.md defines only the three terms local to this package; the rest
  are modern-di's. Its install/inject/register entry reflects real usage
  (install 12, inject 3, register 2), so the three stragglers in factory.py
  and README.md were brought to the winner rather than the entry dropped. One
  is a user-visible ValueError string.
• AGENTS.md gains Workflow and Where-a-fact-goes, loses the lanes, and
  sheds the prose restatement of factory.py that the admission check
  disallows.
• 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).

Why this is chore: and not docs:

release.yml sourced the Release body from planning/releases/<tag>.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 ("drop the planning/ directory and the curated-release-notes
convention", modern-python/modern-di#449), one of the five migration PRs #67
cites. Stable releases lose a hard gate; that is the precedent's call, not a new
one.

Verification

just lint-ci clean. just test-ci passes at 100% coverage, 18 tests
(17 plus the new public-surface invariant). No remaining relative link points
into planning/ or architecture/; the new offline lychee job covers the rest.

Rebased onto 11a0d11 for this PR. It picked up the three README/expose()
changes that landed while it sat (#37, #38, #39) with no conflicts, and the two
independent edits to the same expose() paragraph composed correctly: this
branch's install/onto rewording plus #39's TypeError sentence.