fix: restore workspace unsafe_code=forbid, fix TaskClaim TOCTOU, remove TUI deps

Review fixes for PR ultraworkers#3250:

1. Restore workspace-level unsafe_code = "forbid" (was downgraded to
   "warn" for TUI support). The per-crate [lints.rust] unsafe_code =
   "allow" on rusty-claude-cli already overrides it where needed.

2. Fix TaskClaim TOCTOU race in claim_task(). Replace exists()-then-write
   with OpenOptions::create_new(true) which atomically fails if the lock
   file already exists, eliminating the race window between the check and
   the write.

3. Remove TUI scaffolding deps (ratatui, tui-textarea, gag, crossbeam-channel,
   libc, unicode-width) and the /tui slash command spec — these belong in
   a future TUI PR and are unused in the current source.

Also updates the provider chain precedence test to reflect the new behavior
where the caller's resolved model is the chain primary.

Co-Authored-By: Claw <noreply@openclaw.ai>

Author: TheArchitectit