## Why

The `os-adr` plugin's two hardest requirements — unprompted write-trigger recognition and
unprompted correct retrieval (`04-plugin-requirements.md` reqs 4–5) — have never been measured.
Eval A (`plugins/os-adr/eval/`) only proves the plugin executes correctly once explicitly
invoked; it says nothing about whether an agent notices, on its own, that the ADR system is
relevant. The scenario *shapes* for this held-out eval already exist
(`docs/adr-system/06-eval-scenarios.md`) but no fixtures, prompts, checker, or runner exist yet.
Building it now closes the last unmeasured requirement in the locked rollout order.

## What Changes

- Build a new eval harness, `plugins/os-adr/eval-b/`, sibling to but structurally distinct from
  Eval A, for the 7 held-out scenarios (W1–W3 write-trigger, R1–R4 retrieval).
- Author full scenario prompts/fixtures from the frozen shapes in `06-eval-scenarios.md` —
  scenario *shapes* are not renegotiated by this change; only their execution artifacts are new.
- Fixture project(s): a realistic, onboarded project state (populated `docs/adr/` incl. a
  Superseded pair for R2 distractors, a built `graphify-out/` for R4) that the SessionStart hook
  recognizes as ADR-initialized.
- A **headless runner** (real `claude -p` in a real sandbox cwd) as the primary execution mode,
  not an in-session Agent-tool subagent — because the SessionStart hook firing for real is part
  of what's under test (Eval A's subagent shortcut is not valid here; see design.md).
- A deterministic-first, two-axis checker: (a) did the agent consult/propose the ADR system
  unprompted at all, (b) did it act on/write the *specific correct* thing. Axis (a) is
  mechanically checkable (tool-call/transcript grep for `/os-adr:*` or `docs/adr/` reads/writes);
  axis (b) requires a small LLM-judged correctness check against the fixture's known-correct
  answer, kept as thin and rubric-bound as possible.
- Sandbox/grid tooling mirroring Eval A's shape (`bin/sandbox`, `bin/check`, results.tsv) adapted
  for headless execution and the two-axis score.
- Wiring for the later `autoresearch` iteration stage (not run by this change — building the
  harness only).

Out of scope for this change: actually running the eval grid, iterating retrieval/trigger
heuristics, or adding/altering scenario shapes in `06-eval-scenarios.md`.

## Capabilities

### New Capabilities
- `adr-eval-b-harness`: fixtures, held-out scenario prompts, headless sandbox/runner tooling, and
  a two-axis deterministic-first checker for the unprompted write-trigger and retrieval eval.

### Modified Capabilities
(none — this adds a new, separate eval surface; it does not change the behavior of
`adr-authoring`, `adr-retrieval`, `adr-session-awareness`, or any shipped plugin capability)

## Impact

- New directory: `plugins/os-adr/eval-b/` (fixtures, scenarios, bin/, runner assets, README).
- No changes to plugin source (`lib/adr/`, `bin/adr-*`, `hooks/`, `skills/`) — this is a pure
  test-harness addition.
- Depends on `claude` CLI headless mode (`claude -p`) being available in the build/dev
  environment for the runner to execute against a real sandbox with hooks firing.
- Once built, unlocks the deferred final stage of the locked os-adr rollout order (running the
  grid + `autoresearch` iteration), tracked separately in `CLAUDE.md`.