feat: implement Ctrl+Space prefix key (#35)

* feat: implement Ctrl+Space prefix key for mux commands

- Add Ctrl+Space as the single prefix/leader key
- Prefix commands: h/j/k/l focus, c new tab, x close, n/p tabs, m monitor, g home, ? help, q quit
- Implement copy mode with [ (j/k scroll, g/G top/bottom, q exit)
- Remove Ctrl key interceptions from terminals (Ctrl+W, Ctrl+U, etc now passthrough)
- Add PREFIX indicator overlay when prefix mode is active
- Update all help text to reflect new keybindings
- Esc cancels prefix mode without forwarding

Amp-Thread-ID: https://ampcode.com/threads/T-019b9846-e370-726f-9c8b-7e7111b644ea
Co-authored-by: Amp <amp@ampcode.com>

* chore(ux): polish prefix/monitor modes per review
- Monitor mode now acts as a true chooser: Enter selects/opens, q/Esc cancels.
- Added visible 'COPY MODE' indicator (orange) to center and sidebar panes.
- Copy mode now supports Esc to exit and Ctrl+u/d for half-page scrolling in sidebar.
- Fixed monitor header text and missing prefix indicator in monitor view.
- Updated dashboard help to correctly display prefix commands.

* chore(ux): update stale hotkey references
- Replaced 'Ctrl+T' with 'C-Space c' in welcome screen and empty state.
- Replaced '?' with 'C-Space ?' in welcome screen.

* chore(ux): update prefix keybindings
- New agent tab: C-Space c -> C-Space a
- Focus Up: C-Space k -> C-Space u
- Focus Down: C-Space j -> C-Space d
- Updated all UI references and help text.

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Co-authored-by: Amp <amp@ampcode.com>

Author: andyrewlee