Add two-tier model config integration for LLM cost control

[G9Pedro]

Summary

• Added managed config keys for tiered model selection in src/lib/config-manager.ts:
  • models.background
  • models.default
  • models.complex
• Extended managed config defaults/schema to include a models section while preserving all existing keys and behavior.
• Added coercion/validation support for the new models.* keys and ensured resetConfig clears tiered model overrides.
• Updated observer compression model resolution so that when observer.compression.model is not set, it uses models.background as the fallback model.

Scope safety

• No existing features were removed.
• No existing config keys were removed.
• Search backends, fact extraction, embeddings, and provider support remain intact.

Tests

• Added/updated tests in existing suites:
  • src/lib/config-manager.test.ts
  • src/observer/observer.test.ts
• Verified focused suites:
  • npm test -- src/lib/config-manager.test.ts src/lib/llm-provider.test.ts src/lib/llm-adapter.test.ts src/observer/observer.test.ts
• Verified full suite:
  • npm test

All tests pass.