design: compression eval harness — add three scrubbed fixtures + scrubber

Adds scripts/compression_eval/ with a design doc, README, a placeholder
run_eval.py, and three checked-in scrubbed session fixtures. No working
eval yet — PR is for design review before implementation.

Motivation: we edit agent/context_compressor.py prompts and
_template_sections by hand and ship without any automated check that
compression still preserves file paths, error codes, or the active task.
Factory.ai's Dec 2025 write-up
(https://factory.ai/news/evaluating-compression) documents a probe-based
eval scored on six dimensions. We adopt the methodology; we do not publish
scores.

Contents:
- DESIGN.md — fixture format, probe format (recall / artifact /
  continuation / decision), six grading dimensions, report format,
  cost expectations, scrubber pipeline, open questions, and staged
  follow-up PR plan.
- README.md — short 'what this is / when to run it' page.
- run_eval.py — placeholder that prints 'not implemented, see
  DESIGN.md' and exits 1.
- scrub_fixtures.py — reproducible pipeline that converts real sessions
  from ~/.hermes/sessions/*.jsonl into public-safe JSON fixtures.
  Applies: redact_sensitive_text, username path normalization, personal
  handle scrubbing, email and git-author normalization, reasoning
  scratchpad / <think> stripping, platform user-mention scrubbing,
  first-user paraphrase, system-prompt placeholder, orphan-message
  pruning, and tool-output size truncation for fixture readability.
- fixtures/feature-impl-context-priority.json — 75 msgs / ~17k tokens.
  Investigate → patch → test → PR → merge.
- fixtures/debug-session-feishu-id-model.json — 59 msgs / ~13k tokens.
  PR triage + upstream docs + decision.
- fixtures/config-build-competitive-scouts.json — 61 msgs / ~23k tokens.
  Iterative config accumulation (11 cron jobs across 7 weekdays).

PII audit: zero matches across the three fixtures for the maintainer's
handle (all case variants), personal email domains, and known contributor
emails. Only 'contributor@example.com' placeholder remains.

Why scripts/: requires API credentials, costs ~\$1 per run, LLM-graded
(non-deterministic), must not run in CI. scripts/sample_and_compress.py
is the existing precedent for offline credentialed tooling.

scripts/compression_eval/README.md

@@ -0,0 +1,59 @@
+# compression_eval
+
+Offline eval harness for `agent/context_compressor.py`. Runs a real
+conversation transcript through the compressor, then probes the
+compressed state with targeted questions graded on six dimensions.
+
+**Status:** design only. See `DESIGN.md` for the full proposal and
+open questions. `run_eval.py` is a placeholder.
+
+## When to run
+
+Before merging changes to:
+
+- `agent/context_compressor.py`
+- `agent/auxiliary_client.py` routing for compression tasks
+- `agent/prompt_builder.py` compression-note phrasing
+
+## Not for CI
+
+This harness makes real model calls, costs ~$1 per run on a mainstream
+model, takes minutes, and is LLM-graded (non-deterministic). It lives
+in `scripts/` and is invoked by hand. `tests/` and
+`scripts/run_tests.sh` do not touch it.
+
+## Usage (once implemented)
+
+```
+python scripts/compression_eval/run_eval.py
+python scripts/compression_eval/run_eval.py --fixtures=401-debug
+python scripts/compression_eval/run_eval.py --runs=5 --label=my-prompt-v2
+python scripts/compression_eval/run_eval.py --compare-to=results/2026-04-24_baseline
+```
+
+Results land in `results/<label>/report.md` and are intended to be
+pasted verbatim into PR descriptions.
+
+## Fixtures
+
+Three scrubbed session snapshots live under `fixtures/`:
+
+- `feature-impl-context-priority.json` — 75 msgs, investigate →
+  patch → test → PR → merge
+- `debug-session-feishu-id-model.json` — 59 msgs, PR triage +
+  upstream docs + decision
+- `config-build-competitive-scouts.json` — 61 msgs, iterative
+  config accumulation (11 cron jobs)
+
+Regenerate them from the maintainer's `~/.hermes/sessions/*.jsonl`
+with `python3 scripts/compression_eval/scrub_fixtures.py`. The
+scrubber pipeline and PII-audit checklist are documented in
+`DESIGN.md` under **Scrubber pipeline**.
+
+## Related
+
+- `agent/context_compressor.py` — the thing under test
+- `tests/agent/test_context_compressor.py` — structural unit tests
+  that do run in CI
+- `scripts/sample_and_compress.py` — the closest existing script in
+  shape (offline, credential-requiring, not in CI)