chore(p0): remove the improve lever and the plane improvement loop

Off every active line (PATHFINDER-2026-10-13/03-unified-proposal.md, cuts).
Deleted: src/openultrasast/improve/ (1,175 lines), the ousast improve
subcommand, plane/tasks/loop.py and loop.yaml with the --loop generation,
the memory-snapshot step in memory.seed, pairs.build_pair_signals, and the
four improve tests plus the loop cases in other tests. Docs no longer
describe the lever. Net: 3,073 lines deleted, 83 added. Targeted tests:
154 passed; 3,436 tests collect.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

docs/threat-model.md

@@ -14,10 +14,8 @@ It also lists the controls that keep a scan safe to run in CI and safe to share.
   - Only `deep` mode executes target-derived code. Its REGRESS stage loads promoted candidates
     inside the Docker sandbox described below. It runs only when `docker
     info` succeeds (otherwise the stage is skipped and recorded as `sandbox_unavailable`).
-- **The ruleset and CWE policy are trusted, governed data.** They change in only two ways:
-  through reviewed commits, or through the bounded self-improvement loop (`ousast improve`). The
-  loop can flip rule *status* and tune score constants. It can never edit pattern text or the
-  authoritative 0–5 severity. See [architecture.md](architecture.md#self-improvement).
+- **The ruleset and CWE policy are trusted, governed data.** They change
+  through reviewed commits.
 - **Provider credentials are the operator's.** The core install (PyYAML only) makes no network
   calls on its own.
   - A local scan calls a model only in standard or deep mode, and only when a key is available.