agent/tools, llm/errors: add validation circuit breaker and retryable tool_use mismatch

[thewoolleyman]

Summary

• Add a circuit breaker to the tool validation loop that stops retrying after repeated identical tool_use mismatch errors, preventing infinite retry loops
• Make tool_use block ID mismatches retryable at the LLM error layer so transient streaming corruption can self-heal
• Includes design plan in docs/plans/

Test plan

• Unit tests for circuit breaker trip logic in tool_registry_test.go
• Unit tests for retryable error classification in errors_test.go
• Full test suite passes on macOS and Linux CI

🤖 Generated with Claude Code

[thewoolleyman]

Note: After merging this PR, #41 (test fixes + gofmt + CI) should be merged last — it includes a repo-wide gofmt pass that touches files in this PR. Merging #41 after the others avoids conflicts.