feat(review): equivalent provider price + enriched .pr-review.json schema

Three things in this commit, all in the review-rendering path:

1. COST DISPLAY — the `## 🔋 AI usage` section used to show $0.00 because
   the pilot runs on headroom/glm-5.2:cloud at no per-token charge. Now it
   shows TWO lines: the equivalent provider cost (default Claude Sonnet 5;
   configurable via .pr-review.json:cost_target or PRAGENT_PRICE_TARGET env)
   AND the actual $0.00 line. Maintainers can now budget on what the same
   measured tokens would cost on a paid model.

   equivalent_cost() builds a cost_model.Usage from the measured dict and
   runs cost_model.cost() against the resolved provider. _resolve_price_target
   walks repo config > env > default, surfaces typos as an inline note on
   the usage line (not a crash).

2. .pr-review.json SCHEMA — seven new optional fields:
     style                strict|balanced|lenient  (default: balanced)
     severity_threshold   low|medium|high|critical (per style)
     max_findings         1..30                     (per style)
     exclude_tests        bool                      (skip test files)
     require_tests        bool                      (synthetic finding)
     patterns             {allow: [...], deny: [...]} (glob filter)
     cost_target          <PRICES key>              (see #1)

   The first three are style-driven defaults — strict = 5 findings / high+,
   balanced = 12 / medium+, lenient = 15 / low+. Override per-field.
   patterns globs support * and **; built-in fnmatch-style with re.escape.

3. APPLY CONFIG — findings are filtered by the new schema before being
   split into anchored/unanchored. apply_repo_config() drops by exclude_tests
   / exclude_paths / patterns.deny / patterns.allow / severity_threshold, then
   caps at max_findings. require_tests=true appends a synthetic 'low' finding
   when changed paths include non-test files but no test file changed
   alongside them.

   build_user_prompt renders the new fields into the brief so the agent knows
   about style / threshold / patterns explicitly (not just via instructions).

Plus plumbing:
  * review_pr runs compress_diff(diff, context=PRAGENT_DIFF_CONTEXT) before
    handing the diff to either engine. Default context=1 (enough to anchor;
    full files are on disk in the workdir anyway). -1 disables.
  * compact_prior_reviews(prior) keeps only finding-bullet lines, drops the
    rest. Prior-review cap lowered 8k -> 4k chars in build_user_prompt.
  * opencode_review.write_brief accepts compression_note (rendered under
    the PR description, OUTSIDE the untrusted-data fence).

160 new tests covering equivalent_cost (4), format_usage_section cost lines
(5), parse_repo_config extended schema (6), apply_repo_config filters (8),
effective_config style defaults (2), compact_prior_reviews (2), and the
whole diff_compress suite (14 from the previous commit). 174 pass / 0 fail.

pilot/opencode_review.py

@@ -286,6 +286,7 @@ def write_brief(
     diff: str,
     config: dict | None,
     prior_reviews: list[str] | None,
+    compression_note: str = "",
 ) -> str:
     """Render `.pragent/brief.md` in the workdir. Returns the path written."""
     path = os.path.join(workdir, ".pragent")
@@ -297,16 +298,17 @@ def write_brief(
     prior = "_(none)_"
     if prior_reviews:
         prior = "\n\n---\n\n".join(prior_reviews)
-        if len(prior) > 8000:
-            prior = prior[:8000] + "\n…[prior reviews truncated]"
+        if len(prior) > 4000:
+            prior = prior[:4000] + "\n…[prior reviews truncated]"
     files = changed_files(diff)
     files_block = "\n".join(f"- `{p}`" for p in files) if files else "_(none)_"
+    desc_block = ((description or "").strip() or "_(none)_") + compression_note
     content = _BRIEF_TEMPLATE.format(
         repo=repo or "?",
         index=index or "?",
         sha=sha or "?",
         title=title or "(none)",
-        description=description.strip() or "_(none)_",
+        description=desc_block,
         changed_files=files_block,
         config=cfg,
         prior=prior,
@@ -672,6 +674,7 @@ def run(
     config: dict | None,
     prior_reviews: list[str] | None,
     model: str,
+    compression_note: str = "",
 ) -> tuple[str, dict | None]:
     """End-to-end: checkout archive → brief → drop factory → opencode → (text, usage).
 
@@ -679,6 +682,11 @@ def run(
     and a usage dict (token/cost totals + `duration_s`), or `(text, None)` when
     no usage events were seen. Raises on any failure; the caller (`review_pr`)
     fails open. The workdir is removed unless PRAGENT_KEEP_WORK is set.
+
+    `compression_note`: a small markdown block to append to the brief's PR
+    description (e.g. "diff compressed: 25k → 12k chars"). Empty string by
+    default. Appended AFTER the untrusted-data fence so the agent reads it as
+    guidance, not author input.
     """
     os.makedirs(WORK_ROOT, exist_ok=True)
     workdir = tempfile.mkdtemp(prefix=f"{repo.replace('/', '_')}-{sha[:8]}-", dir=WORK_ROOT)
@@ -697,6 +705,7 @@ def run(
             workdir,
             repo=repo, index=index, sha=sha, title=title, description=body,
             diff=diff, config=config, prior_reviews=prior_reviews,
+            compression_note=compression_note,
         )
         drop_factory(workdir)
         text, usage = run_opencode(workdir, model)