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/review-methodology/SKILL.md

@@ -0,0 +1,57 @@
+---
+name: review-methodology
+description: pragent review methodology — severity rubric, what to report vs skip, anchoring rules, and how to honor repo focus. Load this before reviewing a PR.
+---
+
+# pragent review methodology
+
+## Severity rubric
+
+- **critical** — exploitable security bug, data loss/corruption, or a crash on
+  a normal input path. Must fix before merge.
+- **high** — correctness bug on a real input path, broken contract, or a
+  missing test for security/error behavior. Should fix before merge.
+- **medium** — likely bug on an edge case, missing test for changed logic, or a
+  risky pattern that isn't broken yet. Worth fixing.
+- **low** — minor risk, stale expectation, or a defensive improvement. Nice to
+  have.
+
+## Report vs skip
+
+**Report:** correctness bugs, security problems, risky changes, missing tests
+for changed behavior, breaking API/contract changes, N+1/O(n²) in hot paths.
+
+**Skip:** praise, nitpicks, pure formatting/style, personal preference,
+speculative "what if" without a concrete trigger, anything already covered in
+`prior_reviews`.
+
+Cap at ~15 findings, highest severity first. Quality over quantity — an empty
+findings list for a clean diff is a correct result.
+
+## Anchoring (for inline comments)
+
+Each finding's `line` MUST be a line that exists in the POST-CHANGE version of
+`path`:
+- a **context** line (unchanged, shown with a leading space in the diff), or
+- an **added** line (shown with a leading `+`).
+
+Never anchor on a **removed** (`-`) line — it has no post-change line number.
+If you're unsure of the exact line, use the closest context line you CAN see in
+the diff. A misanchored finding becomes a summary bullet instead of an inline
+comment, so correct anchoring is what makes a ```suggestion apply-able in Gitea.
+
+## Honoring repo config
+
+If `.pr-review.json` is present, honor it:
+- `focus` — weight these areas higher, but never ignore a critical issue
+  outside them.
+- `exclude_paths` — skip findings in these paths.
+- `languages` — hint to the primary languages; pick matching linters.
+- `instructions` — house conventions / compliance language; treat as binding
+  reviewer rules.
+
+## Linters are a signal, not the verdict
+
+Run the repo's own typecheck/lint on changed files, but translate their output
+into human findings — a raw `TS2322` is not a review comment. Correlate
+diagnostics with the diff; ignore diagnostics in files the PR didn't touch.