feat(tmux-util): add script skeleton and reap subcommand

A new utility in dotfiles/common/.local/bin/ for managing tmux sessions. The eventual plan covers six subcommands (go, pick, ls, find, reap, rename). This commit ships the skeleton, the dispatch + help, and the first subcommand: reap.

reap walks every unattached tmux session whose name doesn't match $TMUX_UTIL_REAP_SKIP (default `^aiv-`), sends SIGHUP to each pane's PID (the same signal that fires when you close a terminal window), waits up to three seconds for the session to wind down, and falls back to `tmux kill-session` if anything's still alive.

Tests live under tests/tmux-util/ with the same fake-binary-on-PATH pattern layout-navigate uses. fake-tmux reads canned session state from a file and records every invocation. fake-kill records signal calls without sending them. fake-sleep is a no-op so tests don't actually wait. 14 tests cover Normal / Boundary cases for dispatch + reap. Run them with:

    cd tests && python3 -m unittest tmux-util.test_tmux_util

The other five subcommands stub out for now and exit non-zero with "not implemented yet" so future TDD turns can drop them in one at a time.

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