feat(cli): add /add-dir to track extra directories alongside cwd

[emal-avala]

Summary

Adds `/add-dir` — lets the user extend the session's working set beyond `cwd` without restarting. Tracked dirs are surfaced in the system prompt so the agent knows it's allowed to read and edit there without re-asking on every cross-directory operation.

Forms:

• `/add-dir` — list tracked extras
• `/add-dir ` — add one (path must exist; stored canonicalized)
• `/add-dir --remove ` — remove one
• `/add-dir --clear` — remove all

Also surfaces the list in `/context`.

Why

Today, when you're working on a mono-repo worktree and want the agent to reference a sibling worktree (or a standalone package outside the repo), you either have to restart in the new dir or hand-wave file paths at the agent. This makes the boundary explicit.

Implementation

• New `additional_dirs: Vec` on `AppState` (session-scoped, not persisted)
• `build_system_prompt` injects the list under `# Environment` when non-empty
• Paths are canonicalized on add so the agent sees unambiguous absolute paths

Test plan

• `cargo check` passes
• `cargo clippy --no-deps` clean
• `cargo test -p agent-code-lib --lib query` (2 new tests: prompt omits block when empty, lists entries when populated)
• `cargo test -p agent-code-lib --lib state` (default assertion added)
• Manual: `/add-dir /tmp` adds and shows in `/context`
• Manual: `/add-dir /nonexistent` errors cleanly
• Manual: `/add-dir --remove /tmp` removes, `--clear` empties

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