feat(tmux-util): add rename subcommand (fzf pick + prompt)

tmux-util rename closes out the original six-subcommand plan. The flow:
1. fzf-pick a session from the list.
2. Prompt for a new name on stdin.
3. Bail with a useful message on empty input, same-as-old, or conflict with an existing session.
4. Otherwise `tmux rename-session -t <old> <new>` and confirm.

The conflict check uses `tmux has-session -t =<new>` with the same `=`-prefix exact-match guard as the go subcommand. Without it, tmux's default prefix matching would let `rename foo` succeed even when a session named `foobar` already exists, then surprise the user later.

5 new tests cover Normal cases (pick + rename happy path) and Boundary cases (no sessions, fzf cancel, empty new name, same-as-old no-op, conflict with existing session). The test harness's run_script grew an `stdin=` param so tests can feed the prompt input. fake-tmux picked up a rename-session handler that mutates the state file. Total suite: 48 tests, all green.

Six subcommands shipped: go, pick, ls, find, reap, rename. The original "no args prints help" requirement still holds, and the stub-test for unimplemented subcommands got removed since everything's wired now.

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