docs(harnessx-removal): the plane replaces HarnessX in README, docs and specs (task 8)

README: "Configuring the HarnessX agentic plane" becomes "The agentic
plane (ax)" (ousast plane run|status|doctor|remember, per-task Model,
budget and egress, pointer to ops/ax/README.md, one tagged retirement
line); "The HarnessX self-improving cycle" becomes "The self-improving
cycle", and its MetaAgent paragraph is replaced by the memory proposer
(`ousast improve --memory`, provenance, train-on-test guard). The fusion
panel_model/decider_model lines and the [harnessx] spend caps are gone.
The README reference test now passes without its strict xfail.

docs/examples.md section 6 runs the plane; docs/threat-model.md describes
the per-task EgressPolicy, the usd/calls budgets and the start-request
credential path. RELEASE_NOTES.md gets an Unreleased entry: HarnessX
removed, the retired keys and their errors, the silent loss of the
second hunter for users who set [models] hunter, and the plane as the
agentic path.

Dated retirement notes (2026-09-30) head the requirements, design and
brief of harnessx-self-improving-rulesets, learning-harness,
three-stage-scan and openrouter-sast-harness, and follow the spec table
in steering/overview.md; history is unchanged. All tasks are checked, so
spec.json moves to implementation-complete.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

.kiro/specs/learning-harness/design.md

@@ -1,5 +1,8 @@
 # Design Document: learning-harness
 
+> **Retired 2026-09-30:** HarnessX was removed by `harnessx-removal`; the agentic plane is ax
+> (`ai-service-plane`), and evolutionary proposals come from the plane memory. History below is unchanged.
+
 ## Overview
 
 **Purpose**: Turn OpenUltraSAST from a hand-authored analyzer with a leaking improvement loop into a learning harness: an auto-classifier routes every pair, finding and code region to one of ten verifier-aligned families; one detector configuration per family is evolved one change per round from structured failure facts; a class-aware pair-wise scorer with unscorable denominators and a measured per-family noise floor decides acceptance; only verifier-confirmed claims are reported above suspicion; the split is enforced by construction.