fix(tui): restore agent and model selection parity - #47260

Summary

• Restore V1 agent switching: remember each agent's model and variant within a session, while retaining V2 session-local drafts and durable restoration.
• Fall back to an available model instead of blocking sessions with unavailable selections.
• Honor global and agent model/variant defaults, including live configuration updates and explicit Default overrides.
• Fix recent-model cycling, restore Default in the variant picker, and apply composer selections before custom slash commands.
• Cover CLI overrides, delayed acknowledgments, and same-model agent switches without losing remembered choices or masking command overrides.

Keeps V2 keybindings and session-local variant behavior. No desktop or public API changes. Work started from freshly fetched origin/v2 (211cd73f1a), with fixes split into atomic commits.

Testing

• packages/tui: bun run test
• packages/client: bun test --only-failures
• packages/tui and packages/client: bun typecheck
• Repository pre-push typechecks
• Prettier and git diff --check