feat!: replace the boo ui bottom bar with modals and a command palette

[kylecarbs]

Replaces every bottom-bar interaction in boo ui with centered modals and removes the status bar entirely; the viewport now fills the full terminal height.

What changed

• Palette (C-a): a modal that fuzzy-finds sessions (case-insensitive subsequence match) and runs commands (new session, kill <name>, rename <name>, quit ui, redraw, send a literal C-a). Type to filter, up/down or C-n/C-p to move, enter to run, esc to close.
• Kill (C-a C-k, sidebar x, or palette): confirmation modal, still answered with y/n. Targets are tracked by session name, so a sidebar refresh can no longer redirect a confirmation to a different index.
• Rename (C-a C-r or palette): input modal pre-filled with the current name.
• New session (C-a C-c, + new session, or palette): every create path now opens a name prompt; an empty name keeps the automatic boo new naming (shown as a dim (automatic name) placeholder).
• Bottom bar removed: transient messages (created x, copied 42 characters, errors) render as a self-expiring toast overlaid on the last row instead of occupying a reserved row.
• Docs updated: boo help ui and the README keybinding section.

Breaking change

Printable keys after C-a now feed the palette filter, so C-a k searches for "k" instead of killing. This is what makes fuzzy-find possible, and it removes the old timing dependence (previously the byte after C-a was a command regardless of how stale the prefix was). The GNU-screen-style control variants already documented for boo (C-a C-c/C-a C-k/C-a C-r/C-a C-n/C-a C-p/C-a C-d/C-a C-l, C-a C-a toggle) keep working as direct commands, and a literal C-a is available as a palette command. Plain boo attach bindings are untouched.

Testing

• zig build test-all passes (63 unit + 43 integration tests).
• New unit tests: fuzzy matching, palette filtering, session-gated commands, modal box rendering, updated layout geometry.
• New/updated integration tests: palette open/close + fuzzy attach + palette kill command, rename modal, named and automatic creation via the modal, control-variant bindings, full-height viewport (stty size now reports all rows).
• Manually exercised by driving a nested boo ui with boo send/boo peek: palette, filter, kill/rename/create modals, placeholder, and toasts all render correctly.

Design notes

• Modals are composed per screen row after the normal sidebar/viewport content, the same way the selection highlight already repaints rows, so the existing row-cache diffing handles overlay appearance and disappearance without special cases.
• The palette opens when the C-a prefix arms with no command byte in the same read, mirroring when the old keybind bar appeared. A lone Esc cancelling the armed prefix is swallowed by the input parser, so readTty detects the silent disarm and closes the palette.
• Escape sequences typed while a modal is open are swallowed (Up/Down navigate the palette); previously an arrow key pressed during a rename leaked its final byte into the input buffer.
• Kill/rename modals store the target session name rather than an index into the once-a-second-refreshed session list.

---

This PR was generated by Coder Agents on behalf of @kylecarbs.