docs: stop typing versions the code already knows

Nine documents carried a hand-typed contract version and every one was stale.
One was load-bearing: PLUGIN-EXTERNAL-REPO.md told an external author to pin
">=0.13.0,<0.14.0" while the contract is at 0.19.0, so a plugin built by
following that guide is refused by the loader at startup. The plugin count was
wrong in six more.

They now point at docs/PROJECT-MAP.md, which is generated. A test enforces it:
no document may declare a version literal, a stated current version must match
the code, and a stated plugin count must match the tree. ADRs are exempt from
the current-version rule, because an ADR states the version a decision was taken
AT - that is a record of the past, and rewriting it would falsify the record
ADRs exist to keep.

CONTRACT-STABILITY.md was missing 0.17.0, 0.18.0 and 0.19.0 - including the only
BREAKING change in the series - in the one document a site reads to choose its
pin. All three are recorded, with 0.19.0 called out: it took something away, and
it shipped before it was written down, which is the argument for pinning tight
rather than trusting that a minor bump is safe.

docs/PLUGIN-GUIDE.md

@@ -78,7 +78,8 @@ plugin's identity (ADR-002):
 Two fields deserve attention.
 
 `core_version` is a semver range against the framework's `__contract_version__`
-(declared in `shopdb/__init__.py`, currently `0.13.0`). The loader refuses to load
+(declared in `shopdb/__init__.py`; the current value is in
+[PROJECT-MAP.md](PROJECT-MAP.md), which is generated). The loader refuses to load
 a plugin whose range excludes the running framework. We pin `>=0.6.0` because this
 plugin uses the `get_reports` hook, which was added to the contract in 0.6.0
 (see [PLUGIN-HOOKS.md](PLUGIN-HOOKS.md), "get_reports"). We cap at `<1.0.0` because