Implement local AI modes and guardrails

[dbrosio3]

Summary

• Add first-class AI guardrails to the v2 config schema, types, defaults, templates, and docs.
• Skip only the local AI phase visibly when changed-line or approximate prompt-token budgets are exceeded.
• Thread configurable AI provider timeouts into the Claude adapter and cover mode/guardrail behavior with tests.

Validation

• pnpm test
• pnpm run check:shell

Closes #11.