fix(agent): retry judge schema validation with prettified errors

[caffeinum]

Problem

bu-2-0 occasionally omits required boolean fields (is_correct, verdict) from judge structured outputs entirely. Python upstream silently defaults them to false via pydantic's lax mode; zod hard-rejects and the structured-output parse throws — and operators never learn the model misbehaved.

Observed on a real eval run (codesandbox.com getting-started, 2026-05-07):

WARNING [browser_use.Agent] Simple judge failed with error: [
  {
    "expected": "boolean",
    "code": "invalid_type",
    "path": [ "is_correct" ],
    "message": "Invalid input: expected boolean, received undefined"
  }
]

The previous attempt in this branch defaulted missing booleans to false via a lenientBool helper. That hid the model bug from operators and made it impossible to distinguish "model said false" from "model said nothing."

Approach

Replace the silent default with retry-with-feedback, matching the pattern from PR #34 for action-emission retries:

1. When SimpleJudgeSchema or JudgeSchema fails to parse, send the prettified zod errors back to the LLM and retry up to 2 times.
2. If retries exhaust, surface the failure on the run's final ActionResult so harbor's failure_reason picks it up:
   • _run_simple_judge marks the run as failed with a [Judge schema invalid: ...] note.
   • _judge_trace synthesizes a verdict=false judgement with the schema error in failure_reason.
3. Adds JudgeSchemaInvalidError (src/exceptions.ts) for the internal throw.
4. A first-attempt shape check (any judge-related key present) preserves the prior graceful-skip path when the LLM returns a non-judge JSON shape entirely (e.g. an agent-step JSON in mocked component tests), so we don't regress wired-mock tests.

This pairs strict zod with feedback-driven self-correction instead of papering over the model bug with a default.

Tests

test/agent-judge-schema-retry.test.ts covers:

1. bu-2-0 missing-is_correct retry-then-recover
2. retries exhaust → run marked failed
3. network errors stay swallowed (unchanged behavior)
4. JudgeSchema verdict-missing exhaustion path
5. verdict self-correction on retry

Diff scope

3 files, src + tests only:

• src/agent/service.ts (retry loop + judge schemas)
• src/exceptions.ts (new JudgeSchemaInvalidError)
• test/agent-judge-schema-retry.test.ts

Supersedes the earlier closed iteration of this branch (PR #36).

[caffeinum]

Closing — reverting the silent-skip-on-shape-mismatch logic. Will reopen with cleaner version.