feat(fleet): multi-runtime harness — verifier tests + capped builder shell

The harness layer (builder sessions, cold verifiers, evidence flow) gets a
committable home, per the PRD model-fleet § harness portability and erp#63:

- fleet/harness/verifier/: the two canonical verifier tests (locate-test,
  cold-reader backlog audit) with pinned inputs, verbatim prompts, ground
  truth and pass rules — judged context-free, never self-graded.
- fleet/harness/bin/run-verifier.sh: runs a test against any OpenAI-style
  local endpoint (Ornith/MLX) or vibe -p (Mistral); emits sha256-pinned
  JSON transcripts.
- fleet/harness/bin/vibe-builder.sh: the bounded shell for scoped builders
  and recurring tasks — refuses the trunk (linked-worktree guard), hard
  --max-turns/--max-price caps, full JSON journal per run.
- fleet/README.md layout + AGENTS.md Fleet section updated in the same
  change (same-change freshness rule).

Part of erp#63 (harness portability spike, D2).

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01VRShc4QhLLU73FLHx9vskh

fleet/README.md

@@ -47,7 +47,8 @@ fleet/
 │       ├── prompt.md
 │       └── scripts/
 ├── golden/                    # per-atom golden sets — land with erp#39
-└── profile/                   # fiscal.yaml + calendar.yaml + ADC register — land with erp#54
+├── profile/                   # fiscal.yaml + calendar.yaml + ADC register — land with erp#54
+└── harness/                   # multi-runtime harness layer: verifier tests + builder bench (harness/README.md)
 ```
 
 ## `atom.yaml` — the contract, field by field