feat: give sessions the full height and overlay status content

[kylecarbs]

The last screen row is no longer reserved for a status bar. Sessions in boo ui now get the full terminal height, and the bottom row composes like every other row: sidebar, separator, viewport.

• The Keybinds: Ctrl+A hint moved into the sidebar's bottom row (it fits in the sidebar width), so the separator line runs all the way down.
• Status content (rename prompt, kill confirmation, the armed-prefix keybind list, transient messages) overlays the last row full-width only while it has something to show. When it clears, the row diff cache repaints the row from sidebar and session state, restoring whatever was underneath.
• A session cursor on the last row is hidden while the overlay shows so it does not blink over the overlay text.
• Layout.Hit.status is gone: clicks and drags on the bottom row hit the viewport (or nothing, over the hint/separator), and mouse events there forward to the application like any other viewport row.

This came out of investigating a bottom-bar/cursor positioning issue seen with bottom-anchored TUIs (Claude Code): with a reserved bottom row the session never owned the true last row of the terminal. Removing the reservation removes that interplay entirely; please re-check the original symptom against this branch.

Validation

Dogfooded by running boo ui inside a boo session and peeking it, with an inner session that anchors an input line plus cursor at its last row (Claude Code style):

$ boo peek outer            # bottom row: hint, separator, session content
 Keybinds: Ctrl+A       │claude-input>
$ boo peek outer --json     # cursor sits on the true last row
cursor: {'row': 24, 'col': 40}

Arming C-a overlays the keybind bar across the bottom row; Esc reverts it and the cursor returns to row 24. stty size inside a session now reports the full height (24 75 in a 24x100 terminal).

zig fmt --check, zig build test, and zig build test-integration all pass.

Design notes

• Layout.viewportRows() now returns the full height; every consumer (view sizing, resize propagation, empty-state centering, cursor clamping) follows from that single change.
• The session list capacity is unchanged: the row the status bar used to occupy is now the sidebar hint row, so listRows() keeps the same formula.
• The overlay needs no extra repaint plumbing: arming/disarming the prefix, prompt state changes, and message expiry already set need_render, and the per-row cache diff repaints the bottom row when its composed content changes in either direction.
• Integration coverage: a new waitLastRow helper renders the accumulated PTY output through ghostty-vt and asserts the bottom row's contents, verifying hint+separator, the full-width overlay while armed, and the revert after Esc.

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