feat(config): add disable_title_generation_request and default relocate_third_party_prompts to true

Adds two config flags surfaced in DEFAULT_CONFIG and validateConfig:
- disable_title_generation_request (default true): when set, the
  orchestration helper short-circuits Claude title-generation requests
  locally instead of forwarding them upstream. Stops the background
  Haiku call that was triggering usage toasts and consuming tokens for
  an opencode UI convenience feature most users do not need.
- relocate_third_party_prompts (default true): promotes what was an
  undocumented in-memory flag to a first-class config field with a
  typed getter so integrations can opt out.

Source
- src/config.ts: interface + DEFAULT_CONFIG + validateConfig coverage
  for both flags.

Tests
- tests/unit/config.test.ts: pins the two new defaults and their
  validateConfig passthrough.
- tests/unit/request-orchestration-helpers.test.ts (new): covers the
  title-generation short-circuit path end-to-end, verifying the
  helper never reaches forwardRequest when the flag is enabled and a
  Claude title-generation body is detected.
- tests/integration/plugin/index.parallel.test.ts +
  tests/integration/plugin/session-lifecycle.test.ts: keep their
  DEFAULT_CONFIG mock fixtures in sync with the new shape.

63/63 scoped tests pass.