feat(cli): /cd <path> — change the session working directory

[emal-avala]

Summary

Adds /cd (alias /chdir) for changing the session's working directory mid-session without restarting with --cwd. Matches the shell-builtin feel users expect.

Accepts:

• absolute paths: /cd /home/you/other-project
• paths relative to the current session cwd: /cd ../sibling
• tilde-prefixed paths: /cd ~, /cd ~/Code/foo

No-arg invocation prints the current cwd and usage, so /cd doubles as a "where am I?" probe when you've lost track during a long session.

Implementation notes

• Relative paths resolve against the session cwd, not the process cwd. These can diverge when the user spawns the binary from one dir and then navigates elsewhere — the agent cares about the session cwd.
• Every /cd also calls set_current_dir so subprocess-spawning tools (Bash, PowerShell, REPL) and canonicalize() calls behave consistently. Failure to update the process cwd is a warning, not a fatal error — at worst the agent sees a stale process cwd for one turn.
• reset_system_prompt_cache() is called after the cwd changes so the next turn's # Environment block reflects the new dir.
• ~user expansion is not supported — that would require NSS lookups and isn't portable. Only ~ and ~/ are handled.

Test plan

• cargo clippy --workspace --tests --no-deps -- -D warnings — clean
• cargo test -p agent-code --bin agent commands::tests::resolve_cd_target — 6/6 pass

6 new tests:

1. Empty / whitespace-only input errors
2. Absolute dir path resolves to its canonical form (symlink-aware)
3. Relative path resolves against the supplied base dir
4. Nonexistent path errors
5. Plain-file path errors (not a directory)
6. ~ expands to $HOME (skipped if HOME unset)

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