feat: opencode review engine + .opencode factory

Replace the single Python model-call reviewer with an opencode agent
factory. A primary 'pragent' agent reads a brief (title/body/diff/config/
prior reviews), inspects the checked-out repo, runs the repo's own linters
via bash, loads review-methodology + findings-schema skills, and emits a
{summary, findings} JSON with per-finding severity/path/line/problem/fix/
suggestion/reference. Dormant security/tests/perf subagent lenses fan out
only on large/risky diffs (lean by default).

pilot/opencode_review.py: fetches the repo archive at the head sha into a
temp workdir, writes .pragent/brief.md, drops the factory, runs
'opencode run --pure --agent pragent --dir <workdir>' headlessly. Isolates
HOME (shared, warmed), strips ANTHROPIC_* env (leaked host vars caused
ProviderModelNotFoundError), stdin=DEVNULL (opencode blocks on stdin),
maps the bare OLLAMA_MODEL to the provider-prefixed ref. No Gitea I/O —
ai_review.review_pr parses + anchors + posts (reuses all v2 logic/tests).

PRAGENT_ENGINE=opencode (default) selects it; =ollama keeps the legacy
direct-call path. Verified end-to-end: posts a real review with a summary
section, inline [CRITICAL]/[HIGH] comments + apply-able suggestions +
reference links, and the sha dedupe marker. 49 tests pass.

Co-Authored-By: Claude <noreply@anthropic.com>

.opencode/skills/findings-schema/SKILL.md

@@ -0,0 +1,58 @@
+---
+name: findings-schema
+description: The exact JSON output shape pragent must emit at the end of a review. Load this before producing findings.
+---
+
+# pragent findings schema
+
+The review's FINAL message is a short prose summary followed by ONE fenced
+```json code block. The Python shell parses the LAST ```json fenced block in the
+message — so the JSON must be the last thing, and it must be valid.
+
+## Shape
+
+```json
+{
+  "summary": "One-paragraph overview of the change and its risk, plus severity counts.",
+  "findings": [
+    {
+      "severity": "critical|high|medium|low",
+      "path": "path exactly as it appears in the diff's `+++ b/` side",
+      "line": 12,
+      "problem": "one line: what is wrong",
+      "fix": "one line: how to fix it",
+      "suggestion": "exact replacement lines, indented as in the file, or \"\"",
+      "reference": "https://... or \"\""
+    }
+  ]
+}
+```
+
+## Field rules
+
+- `severity` — one of `critical`, `high`, `medium`, `low`. Anything else is
+  coerced to `medium` by the parser.
+- `path` — the post-change path, exactly as in the diff (`+++ b/foo/bar.ts`
+  → `foo/bar.ts`). Required; a finding without a real path is dropped.
+- `line` — a post-change line number (int ≥ 1) that exists in `path` after the
+  PR. Required; bad/missing line → the finding becomes a summary bullet instead
+  of an inline comment.
+- `problem` — one line, concrete: what is wrong and why it matters.
+- `fix` — one line, the remedy. Empty string if the fix is architectural.
+- `suggestion` — the literal new code replacing the flagged line(s). Minimal,
+  just the changed lines, indented as they appear in the file. **Empty string**
+  when no safe textual replacement exists (missing test, architectural note, a
+  fix that needs context beyond one hunk). This is wrapped in a ```suggestion
+  fence → Gitea renders an **apply button**.
+- `reference` — a URL (CVE, library docs, spec) backing the finding, or `""`.
+  Only link authoritative sources; don't fabricate URLs.
+
+## Clean diff
+
+If there's nothing to report: `{"summary":"<what it does, why it's fine>","findings":[]}`.
+
+## Don't
+
+- No prose after the closing ``` of the JSON block.
+- No extra keys — unknown keys are ignored by the parser, so don't rely on them.
+- Don't repeat findings from `prior_reviews`.