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]>

plane/tasks/loop.yaml

@@ -1,18 +0,0 @@
-# loop (harnessx-removal Req 6.3, design section 6): the improvement loop's singletons -- the generator appends the
-# step (snapshot, measure, propose, improve) to the command. No model, budget {usd: 0, calls: 0}; improve replays the
-# benchmark gate unchanged and delivers its verdict as gate.json. The image is digest-pinned.
-apiVersion: ax.io/v1alpha1
-kind: Task
-metadata:
-  name: loop
-  atespace: default
-spec:
-  image: "localhost:5001/ousast-runner@sha256:d2a55293999cc34581ef1f02a1ffb0c2dd8fc0cbe001ac952df81600e868f019"
-  command: ["loop"]
-  resources:
-    requests:
-      cpu: "250m"
-      memory: "512Mi"
-    limits:
-      cpu: "1"
-      memory: "1280Mi"