feat: agent-first prompt flow and plain-text reminders

[ZGFuZHk100]

Summary

• remove mode-first UX in favor of agent-first controls: Tab cycles primary agents, @ sets quick params (agent, model, thinking), and / remains for commands/files
• switch plan/build reminder language to agent terminology and remove system-reminder wrapper formatting from session reminders
• add local pre-release helpers for TUI verification and align docs/tests with the new agent-first behavior

Verification

• bun run pre-release:checks
• bun --cwd packages/0x0 test test/session/llm.test.ts
• bun --cwd packages/0x0 test test/session/system.test.ts
• bun run pre-release:tui (manual plan -> build validation in new session)

[github-actions]

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[ZGFuZHk100]

Closing per request; we will not release this path upstream right now.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found