fix: prevent non-convergent stuck loops on trivial tasks

[AbirAbbas]

Summary

Fixes #12 — Stall/non-convergence on trivial repo_path smoke tasks that exit with code -2.

• Adds _detect_stuck_loop() for the default path (reviewer-only) which previously had no stuck detection (unlike the flagged path which gets it from the synthesizer)
• When stuck or loop-exhausted with non-blocking reviewer feedback AND code changes present, returns COMPLETED_WITH_DEBT instead of FAILED_UNRECOVERABLE — issues flow through the debt gate rather than triggering replanning/abort/stall
• Preserves FAILED_UNRECOVERABLE for genuinely blocking failures and cases where no code was produced

Root cause

The default-path coding loop had no mechanism to detect repetitive non-convergent fix cycles. When a reviewer repeatedly returned approved=False / blocking=False (e.g. requesting minor polish on trivial tasks), the loop burned all 5 iterations + 2 advisor rounds without converging, eventually causing an exit -2 abort with completed_issues: [] and the issue stuck in in_flight_issues.

Behavior change

Scenario	Before	After
3+ consecutive non-blocking fix cycles	Runs all 5 iters → FAILED_UNRECOVERABLE	Breaks at iter 3 → COMPLETED_WITH_DEBT
Loop exhausted, reviewer non-blocking, files changed	FAILED_UNRECOVERABLE	COMPLETED_WITH_DEBT
Loop exhausted, reviewer blocking	FAILED_UNRECOVERABLE	FAILED_UNRECOVERABLE (unchanged)
Loop exhausted, no files changed	FAILED_UNRECOVERABLE	FAILED_UNRECOVERABLE (unchanged)

Test plan

• Unit test: _detect_stuck_loop() with various history patterns
• Integration test: mock coding loop with perpetual non-blocking fix → confirms COMPLETED_WITH_DEBT at iteration 3
• Integration test: blocking reviewer → confirms FAILED_UNRECOVERABLE unchanged
• Integration test: no files changed → confirms FAILED_UNRECOVERABLE unchanged
• Existing test suite passes (test_model_config.py)
• Smoke test: run a trivial repo_path build and verify it converges

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