fix(agents): recover pi JSON output that follows prose

[cjunxiang]

Summary

gnhf --agent pi fails every iteration with Failed to parse pi output: ... is not valid JSON, trips maxConsecutiveFailures, and aborts — even though the model did emit valid JSON. PiAgent was the only adapter parsing its final assistant message with raw JSON.parse; this routes it through the shared parseAgentOutput like every other adapter, recovering JSON that follows a prose summary or is wrapped in fences.

Closes #154.

Two commits:

1. fix(agents): recover pi JSON output that follows prose — the bug fix + regression test.
2. refactor(agents): share parseAgentOutput helper across adapters — extract the now-triplicated parseAgentJson + validateAgentOutput fallback into one parseAgentOutput(text, schema, agentLabel) in types.ts; route pi, opencode, and copilot through it. Net −44 lines.

Root cause

PiAgent was the only built-in adapter that used raw JSON.parse on the final assistant message. acp, copilot, opencode, and rovodev all route through parseAgentJson (src/core/agents/json-extract.ts), which strips Markdown fences and recovers the rightmost balanced JSON object — i.e. it tolerates prose/fences around the JSON.

Pi frequently writes a short prose summary before the JSON object. The final assistant message captured in a real failing run was:

Iteration 3 complete. Established a clean, building petpet foundation ...

{"success":true,"summary":"...","key_changes_made":[...],"key_learnings":[...]}

The leading Iteration 3 complete. makes JSON.parse(finalText) throw Unexpected token 'I', "Iteration "... is not valid JSON, so the iteration is recorded as a failure. Reproduced across three consecutive iterations ("Build gree...", "All work f...", "Iteration "). Verified that parseAgentJson recovers the object from that exact text (all four required keys present, success: true).

This is also why the run appeared "stuck": the guaranteed 3 failures abort the orchestrator, after which the TUI stays open (keepTui = status === "aborted" && isTTY) with the elapsed timer still incrementing, so a stopped run looks still-active until Ctrl+C.

The fix

parseAgentOutput(text, schema, agentLabel) (now in types.ts, next to validateAgentOutput):

1. parseAgentJson(text, validateAgentOutput-as-predicate) — returns the first (rightmost) JSON object that validates.
2. No-predicate fallback parseAgentJson(text) — so schema-invalid JSON still surfaces as Invalid pi output (via a re-thrown validation error) rather than being miscategorized as Failed to parse pi output.
3. throw new SyntaxError(...) when no JSON is present at all → preserved as Failed to parse pi output.

The refactor preserves behavior exactly: the per-agent no-JSON error string is retained via agentLabel, and each adapter keeps its own caller-side error formatting (pi distinguishes Failed vs Invalid; opencode/copilot use a single message). Existing error messages are unchanged:

• "not json" (no object) → Failed to parse pi output ✓
• valid JSON, wrong shape → Invalid pi output ✓

Verification

• tsc --noEmit — clean
• vitest run (unit suite, excluding e2e/) — 619/619 passing, including a new regression test recovers JSON that follows a prose summary
• eslint + prettier --check on changed files — clean