refactor(plugin): retire what the hand-bump scheme left behind — the README that taught it, the floor test, the stale rationale (#3328)

#3127 checklist 19. The step's own deletion list turned out to be largely
spent: `check_version_bump()` came out with #3327, and the machinery the step
expected to delete alongside it is load-bearing for its replacement.

`manifest_version(ref=…)`, `--base`, `--no-version` and the `origin/main`
resolve path all STAY. Derivation makes the value right; it does not make the
comparison unnecessary. `check_version_is_minted` still has to ask "did the
version move when the shipped content did?", and that is a base-branch
question no matter who chose the number. The step was planned before #3327
landed, when the assumption was that these died with the guard.

What was actually still standing, all of it teaching or asserting the retired
scheme:

- `plugin/README.md` told the reader to "set a `version` bump per release."
  A shipped file, instructing the exact act the mint replaced — this is how a
  deleted control gets re-added by someone following the docs. Now says not to
  hand-edit the field, names `make mint-plugin`, and says what a forgotten
  mint costs. (`make` is not installed on every workstation, so the direct
  script invocation is given too.)
- `test_plugin_version_bumped_with_the_hook` asserted `version >= (0,1,31)`
  as a tuple of ints. Under a minted value it passes vacuously — every date
  clears a floor of 0.1.31 — and `int("0415")` silently eats the padding the
  format exists to keep. Superseded by
  `test_the_shipped_manifest_carries_a_minted_version`, which asserts the
  canonical shape instead of an ordering the comparator does not perform.
  Removed whole (rule 22).
- The module preamble still ended on "a written rule that depends on being
  remembered is not a control; this is" — true of the bump guard, and read as
  a stronger claim than the mint can support. Replaced with what the change
  did and did not remove: choosing a number is gone, running the mint is not,
  and the difference is that forgetting is now loud rather than silent.
- An orphaned `# --- the version bump ---` section header with nothing under
  it, and a test docstring still naming `check_version_bump`.

`--no-version` keeps its one legitimate case — on `main` the version is
measured against itself — and now says so in both the usage block and its
`--help`, so it does not read as an escape hatch. `check_session_context_
reports_its_version` stays untouched: a different check with a different job,
and the only thing that makes step 6 readable from a transcript (#2220).

Version minted 2026.09.02.0415 -> 2026.09.02.0438 for the README change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TcCs1CcQ1ormdnzSshKqvN

plugin/README.md

@@ -78,8 +78,13 @@ On install you'll be asked for:
 
 ## Notes
 
-- Set a `version` bump in `.claude-plugin/plugin.json` per release so clients
-  pick up changes.
+- **Do not hand-edit `version` in `.claude-plugin/plugin.json`.** It is minted
+  from the clock — run `make mint-plugin` (or `python3
+  scripts/mint_plugin_version.py`) after changing anything under `plugin/`, and
+  commit the result. The installer decides whether to refresh the cache it
+  executes from by comparing that string, so content that ships without a new
+  version reaches the repo and stops there (#2209). CI fails the lane if you
+  forget.
 - The session-start, auto-inject and prior-art hooks need only a **read**-scoped
   key; the MCP tools need **write** scope to create/update. Every hook is a GET
   for that reason — a read key cannot POST.