[codex] Complete conversation session service

[roackb2]

Summary

• Extend ConversationSessionService with require, latest, and updateSettings so shared session lookup and settings mutation live behind the core chat engine boundary.
• Make default fallback session creation carry engine model/workspace defaults instead of relying on repository-level bare defaults.
• Move TUI session preference and rename paths onto service operations instead of inline persisted-session mutation.
• Export the new session settings input type and document the host-facing session service API.
• Add yarn lint as an alias for the existing yarn eslint command.

Why

The P0 boundary work needs TUI, ask, and web/control-plane surfaces to share the same session semantics. This PR completes the first small service slice without migrating the control-plane service yet, so later host refactors can reuse engine.sessions instead of growing parallel session logic.

Validation

• yarn test:unit src/__tests__/unit/core/conversation-engine.test.ts passed; due the script shape it ran all unit tests: 300 passed.
• yarn typecheck passed.
• yarn test:integration passed outside the sandbox: 261 passed.
• yarn build passed.
• yarn eslint passed.
• yarn lint passed.
• git diff --check passed.

Follow-up

Before web/control-plane migration, the next slice should make the TUI fully rely on ConversationSessionService and add any missing service operations discovered during that inventory.