fix(tui): enable Kitty keyboard protocol on Windows so Shift+Enter inserts newline

Closes #1359. On Windows 11 + VSCode integrated terminal +
PowerShell, pressing `Shift+Enter` in the composer submitted the
message instead of inserting a newline. `Alt+Enter` / `Ctrl+J`
were broken the same way. Root cause: crossterm's
`PushKeyboardEnhancementFlags` checks `is_ansi_code_supported()`
before emitting the escape, and on Windows that helper queries
the console mode rather than the VT capability and
unconditionally returns false — so the Kitty push `\x1b[>1u` was
never written. xterm.js then stayed in legacy mode where
`Shift+Enter` and `Enter` both encode as `\r`, indistinguishable.

The fix writes the push and pop escapes directly under
`#[cfg(windows)]`, bypassing the crossterm capability gate.
VSCode and Windows Terminal honour the Kitty keyboard protocol;
terminals that don't (older conhost without VT processing)
silently discard the unknown escapes. The same gate also meant
`PopKeyboardEnhancementFlags` was silently dropped on Windows in
the `main.rs` panic hook and in
`tui::external_editor::spawn_editor_for_input` — both call sites
now route through `pop_keyboard_enhancement_flags` so a crash or
`$EDITOR` invocation can't leave the parent shell with a
Kitty-enhanced keyboard state.

Two `#[cfg(windows)]` regression tests pin the direct-write path
so accidentally falling back to `execute!()` against
`crossterm::PushKeyboardEnhancementFlags` would now fail in CI:

- `push_keyboard_flags_writes_kitty_push_sequence_on_windows`
- `pop_keyboard_flags_writes_kitty_pop_sequence_on_windows`

Non-Windows behaviour is unchanged — the existing
`recover_terminal_modes_emits_expected_csi_sequences_with_gating`
test still passes on Linux and macOS.

Also adds a v0.8.29 audit note to `docs/KEYBINDINGS.md` and
documents a pre-existing FocusGained stack-depth bug for a
separate fix.

Harvested from PR #1483 by @CrepuscularIRIS

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/KEYBINDINGS.md

@@ -99,6 +99,10 @@ When `[memory] enabled = true`, typing `# foo` and pressing `Enter` appends `foo
 | `y` / `Y`            | Trust the workspace (Trust step)                   |
 | `n` / `N`            | Skip the trust prompt                              |
 
+## v0.8.29 audit notes
+
+- **`Shift+Enter` / `Alt+Enter` newlines now work in VSCode on Windows (#1359).** crossterm's `PushKeyboardEnhancementFlags` command unconditionally returns `Unsupported` on Windows (`is_ansi_code_supported() == false`), so the Kitty keyboard protocol escape was never written to the terminal. Without it, VSCode's xterm.js stays in legacy mode where `Shift+Enter` is indistinguishable from plain `Enter`, causing the composer to send the message instead of inserting a newline. The fix writes the push/pop escapes (`\x1b[>1u` / `\x1b[<1u`) directly on Windows, bypassing crossterm's capability gate. VSCode integrated terminal and Windows Terminal ≥1.17 both honour the Kitty keyboard protocol; terminals that do not understand the sequences silently discard them.
+
 ## v0.8.13 audit notes
 
 - **Ctrl-S is stash, not history search.** Fixed in this revision — `Alt-R` is history search.