feat(tmux-util): add pick subcommand (fzf session switcher)

tmux-util pick lists every session ("attached"/"detached" plus name), pipes it through fzf, and attaches or switch-clients to the chosen one (matching the go subcommand's inside-vs-outside-tmux discipline).

If the user cancels fzf (Ctrl-C, Esc, empty selection), the pipeline returns empty and pick exits 0 without touching tmux state.

The new fake-fzf testing fake is driven by env vars:
- FAKE_FZF_CHOICE_LINE=<N>: return the Nth line of stdin as the selection
- FAKE_FZF_CHOICE=<string>: return the literal string (ignores stdin)
- (neither): exit 130 to simulate cancel

4 new tests cover Normal cases (pick second line, inside/outside tmux behavior) and Boundary cases (no sessions, user cancellation). Total suite: 43 tests, all green.

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