feat(tmux-util): add find subcommand

tmux-util find <pattern> walks every pane across every session, queries each pane's foreground command (`#{pane_current_command}`), and prints the location of any pane whose command matches the pattern. Output format: `<session>:<window>.<pane> <command>`, one per line.

Pattern is a basic ERE (passed through `grep -E`), so anchors and alternation work. Substring matching is the common case.

Exit code:
- 0 with output: matches found
- 1 with no output: no matches (lets you script around it)
- 2 with usage on stderr: missing or empty pattern

7 new tests cover Normal cases (single match, multi-match across sessions, format verification) and Boundary cases (no matches, no sessions, missing pattern, empty pattern). fake-tmux now parses pid:cmd entries in the state file so panes can carry a synthetic command name. Total suite: 39 tests, all green.

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