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-HOOKS.md

@@ -6,11 +6,11 @@ The contract is locked in [ADR-001](../docs/adr/ADR-001-asset-as-platform-contra
 
 ## Contract version
 
-The framework declares its contract version in `shopdb/__init__.py`:
-
-```python
-__contract_version__ = '0.19.0'
-```
+The framework declares its contract version in `shopdb/__init__.py` as
+`__contract_version__`. The current value is in
+[PROJECT-MAP.md](PROJECT-MAP.md), which is generated from the code - this page
+does not restate it, because a version copied into prose is stale within a
+fortnight and a plugin pinned against a stale one is refused at startup.
 
 Each plugin's `manifest.json` declares the range of contract versions it supports: