[codex] Refactor core slash command modules

[roackb2]

Summary

Refactors the M3 slash-command slice so model, auth, compaction, and drift commands live in registered core slash modules instead of the TUI local command switchboard.

Changes include:

• Added module-owned command implementations for /model, /models, /model list, /model set, /auth, /auth status, /auth login, /auth logout, /compact, /drift, /drift status, /drift on, and /drift off.
• Added a TUI slash command context adapter so local-commands.ts can delegate to the core registry while keeping the existing TUI API stable.
• Extended SlashCommandModule with module-owned hints, so help/autocomplete entries for migrated commands no longer need to be duplicated in local-commands.ts.
• Added direct core module tests for command routing, model credential policy, host ports, and module-owned hints.

Impact

This keeps current TUI behavior stable while proving the domain-module command pattern. Future command modules can now own both behavior and help metadata, and local-commands.ts remains focused on compatibility and commands not yet migrated.

Validation

• yarn test:unit src/__tests__/unit/core/slash-command-modules.test.ts
• yarn test:unit src/__tests__/unit/tui/local-commands.test.ts
• yarn test:unit src/__tests__/unit/core/llm-factory.test.ts
• yarn eslint
• yarn typecheck
• yarn test