feat(cli): add /btw for quick-capture notes to user memory

[emal-avala]

Summary

Adds `/btw ` — a one-liner way to save a free-form note to the user's memory directory without going through the model.

• Writes `btw-YYYYMMDD-HHMMSS-.md` via `memory::writer::write_memory` (the same path used by the memory system)
• Slugifies the first ~40 chars for a descriptive filename
• Updates `MEMORY.md` index so the note loads on the next session
• Type: `user` memory · Description: first ~120 chars of the note

Example:
```

/btw prefers short, direct commit messages
Noted. Saved to ~/.config/agent-code/memory/btw-20260422-151200-prefers-short-direct-commit-messages.md
```

Why

When the user says "btw, I always do X" mid-session, the current options are (a) ask the agent to remember it, which costs a turn, or (b) drop out of the REPL to edit the memory file. This bridges that gap.

Test plan

• `cargo check -p agent-code` passes
• `cargo clippy -p agent-code --no-deps` clean
• `cargo test --bin agent commands::tests` (7 unit tests for slugify + truncate)
• Manual: `/btw` with no args prints usage
• Manual: `/btw hello world` creates file and updates MEMORY.md

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