docs(901): the one line a guard author reads names two of the three rules

The record is a rule about the SHAPE of a guard's predicate — the third member
of the set docs/guard-inventory.md:15-16 tells a guard author to read before
editing a guard or adding a row — and that pointer named only
`guard-derives-population-from-source` and `guard-ships-with-mutation-proof`.
Before this commit, `grep -rln guard-pins-the-artifact-not-a-shape docs/`
outside the record and the generated catalog returned docs/README.md alone, so
the record was reachable from the task-signal map and by topic but not from the
inventory a guard author already has open.

That is the reachability failure the record itself names: its body says
`docs/guard-inventory.md` carries its precedent per incident, findable only
from inside one. A record about topic-resolvability that is missing from the
entry point of its own topic reproduces it.

Refs #901

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015QqCpYFsKgnAnx6jVwrKiV

docs/guard-inventory.md

@@ -12,8 +12,10 @@ and `dotnet format`, and `grep -rn pytest .husky/` returns nothing. The runner i
 `docker-build.yml:723` on the release path, which re-runs two of these files
 (`test_ci_dropped_step_guard.py` and `test_ci_release_path_scan_job.py`) as a `needs:` of `build`.
 
-**Read `docs/decisions/records/testing/guard-derives-population-from-source.md` and
-`…/guard-ships-with-mutation-proof.md` before editing a guard or adding a row.**
+**Read `docs/decisions/records/testing/guard-derives-population-from-source.md`,
+`…/guard-ships-with-mutation-proof.md` and — for the SHAPE of the predicate itself, whenever it
+decides by reading shell or config TEXT — `…/guard-pins-the-artifact-not-a-shape.md` before editing
+a guard or adding a row.**
 
 ## Columns