agent search: spotlight-style fuzzy finder on ctrl+opt+cmd+shift+u

a centered, non-activating key panel that lists every agent and jumps to
the chosen one through the same dingger focus path as a row click.

- fzf-style matching: each word must match one of name, session, window,
  harness, status, or directory, letters in order; the best alignment wins
  (word starts and consecutive runs score higher), matched title letters
  are highlighted.
- empty query keeps the menu's order, so enter jumps to the top waiting
  agent.
- the selection follows the agent, not the position, as states change;
  the list scrolls to keep it visible.
- keys: arrows, ctrl+n/p, ctrl+j/k, enter, esc, ctrl+u; the shortcut again
  or clicking elsewhere closes it.
- search_hotkey setting; an unedited config file is upgraded. menu item
  "Search Agents...". rows carry the pane's directory.
- --print-search and --snapshot-search debug flags; the panel's background
  and snapshot code are shared.

selftest covers matching, ranking, highlighting, and selection; e2e step
15 checks queries through the real binary (fails when search only looks at
titles). e2e resolves relative binary paths, which tmux hooks need.

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