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/API-REFERENCE.md

@@ -75,8 +75,8 @@ keying) are covered there as well.
 
 Everything else is the core UI API: the endpoints the Vue frontend calls. As a
 rule these are JWT-authenticated (a login token or a managed Personal Access
-Token) and versioned by the plugin contract (`__contract_version__`, currently
-0.16.0). Behavior and stability guarantees are in **CONTRACT-STABILITY.md**;
+Token) and versioned by the plugin contract (`__contract_version__`; the
+current value is in [PROJECT-MAP.md](PROJECT-MAP.md), which is generated). Behavior and stability guarantees are in **CONTRACT-STABILITY.md**;
 sister sites should pin tight `core_version` ranges until the contract reaches
 1.0.