feat(cli): recognise Shift+Enter as a newline key

Closes #5346.

Most terminals send the same byte sequence for `Enter` and `Shift+Enter`
by default, so the application can't tell them apart — this is a terminal
protocol limitation, not something Hermes can paper over. But terminals
that implement the Kitty keyboard protocol (Kitty / foot / WezTerm /
Ghostty by default; iTerm2 / Alacritty / VS Code terminal / Warp once the
protocol is enabled) DO emit a distinct sequence for `Shift+Enter`:

  - `\x1b[13;2u`     — Kitty / CSI-u, modifier=2
  - `\x1b[27;2;13~`  — xterm modifyOtherKeys=2

Stock prompt_toolkit doesn't have the CSI-u sequence in its
`ANSI_SEQUENCES` table at all, and it maps the modifyOtherKeys variant to
plain `Keys.ControlM` (Enter) — i.e. it strips the Shift modifier, which
is the bug users actually hit on iTerm2 and friends.

This PR adds `hermes_cli/pt_input_extras.install_shift_enter_alias()`,
called once at CLI startup from `cli.py`, which inserts/overwrites those
sequences in `ANSI_SEQUENCES` so they decode to `(Keys.Escape, Keys.ControlM)`
— the same key tuple `Alt+Enter` produces. The existing Alt+Enter newline
handler (`@kb.add('escape', 'enter')` in `cli.py`) then fires unchanged,
so there is no new keybinding to register and no behavioral change for
terminals that don't emit the distinct sequences.

Files
=====

* `hermes_cli/pt_input_extras.py` — new module hosting the helper. Lives
  outside `cli.py` so it's importable in tests without dragging in the
  full CLI runtime (which depends on `fire`, `rich`, etc.).
* `cli.py` — calls `install_shift_enter_alias()` once at module import.
  Wrapped in try/except so prompt_toolkit version drift can't break CLI
  startup.
* `tests/cli/test_cli_shift_enter_newline.py` — 6 tests:
  - registration of all three byte sequences
  - overwrite of stock prompt_toolkit's broken modifyOtherKeys mapping
  - idempotency
  - parser equivalence: CSI-u Shift+Enter == Alt+Enter
  - parser equivalence: modifyOtherKeys Shift+Enter == Alt+Enter
  - plain Enter remains a single key (submit), distinct from the two-key
    Alt+Enter / Shift+Enter tuple
* `website/docs/user-guide/cli.md` — keybinding table updated; new
  "Shift+Enter compatibility" subsection with a per-terminal status table
  noting macOS Terminal / stock Windows Terminal cannot distinguish the
  keystroke at the protocol level.
* `website/docs/getting-started/quickstart.md`,
  `website/docs/guides/tips.md` — short mention pointing readers at the
  full compatibility note in `cli.md`.

Tested
======

  pytest tests/cli/test_cli_shift_enter_newline.py        # 6 passed

Live-tested by triggering `\x1b[13;2u` against the running Vt100Parser
(see test). Not exercised in a real terminal end-to-end because that
requires a Kitty-protocol-capable host; the test exercises the parser
path that drives the live terminal too.

tests/cli/test_cli_shift_enter_newline.py

@@ -0,0 +1,88 @@
+"""Verify Shift+Enter byte sequences parse to the same key tuple Alt+Enter
+produces, so the existing Alt+Enter newline handler in `cli.py` fires for
+terminals that emit a distinct Shift+Enter under the Kitty keyboard protocol
+or xterm modifyOtherKeys mode.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from prompt_toolkit.input.ansi_escape_sequences import ANSI_SEQUENCES
+from prompt_toolkit.input.vt100_parser import Vt100Parser
+from prompt_toolkit.keys import Keys
+
+from hermes_cli.pt_input_extras import install_shift_enter_alias
+
+
+SHIFT_ENTER_SEQUENCES = (
+    "\x1b[13;2u",      # Kitty / CSI-u, modifier=2 (Shift)
+    "\x1b[27;2;13~",   # xterm modifyOtherKeys=2
+    "\x1b[27;2;13u",
+)
+
+
+@pytest.fixture(autouse=True)
+def _ensure_alias_installed():
+    """Make every test idempotent — install the alias once per test run."""
+    install_shift_enter_alias()
+
+
+def _parse(byte_seq: str):
+    out = []
+    parser = Vt100Parser(out.append)
+    for ch in byte_seq:
+        parser.feed(ch)
+    parser.flush()
+    return [kp.key for kp in out]
+
+
+def test_install_registers_all_three_sequences():
+    for seq in SHIFT_ENTER_SEQUENCES:
+        assert seq in ANSI_SEQUENCES, f"missing mapping for {seq!r}"
+        assert ANSI_SEQUENCES[seq] == (Keys.Escape, Keys.ControlM)
+
+
+def test_install_overwrites_stock_modifyotherkeys_shift_enter():
+    """Stock prompt_toolkit maps `\\x1b[27;2;13~` to plain Keys.ControlM —
+    i.e. it drops the Shift modifier and treats Shift+Enter like Enter,
+    which is the bug this helper exists to fix. The install must overwrite
+    that entry."""
+    seq = "\x1b[27;2;13~"
+    ANSI_SEQUENCES[seq] = Keys.ControlM
+    install_shift_enter_alias()
+    assert ANSI_SEQUENCES[seq] == (Keys.Escape, Keys.ControlM)
+
+
+def test_install_returns_zero_when_already_correct():
+    """Idempotency — running install twice should not report a second change."""
+    install_shift_enter_alias()
+    assert install_shift_enter_alias() == 0
+
+
+def test_csi_u_shift_enter_parses_as_alt_enter():
+    """Kitty keyboard protocol Shift+Enter must parse to the same key tuple
+    Alt+Enter produces, so the existing handler is reused."""
+    alt_enter = _parse("\x1b\r")
+    shift_enter = _parse("\x1b[13;2u")
+    assert shift_enter == alt_enter, (
+        f"Shift+Enter via CSI-u should parse identically to Alt+Enter; "
+        f"got {shift_enter!r} vs {alt_enter!r}"
+    )
+
+
+def test_modify_other_keys_shift_enter_parses_as_alt_enter():
+    """xterm modifyOtherKeys=2 Shift+Enter must parse identically to Alt+Enter."""
+    alt_enter = _parse("\x1b\r")
+    shift_enter = _parse("\x1b[27;2;13~")
+    assert shift_enter == alt_enter
+
+
+def test_plain_enter_remains_distinct_from_alt_enter():
+    """Plain Enter must keep emitting a single key (submit), not a two-key
+    Alt+Enter tuple — otherwise we would have broken submit."""
+    enter = _parse("\r")
+    alt_enter = _parse("\x1b\r")
+    assert enter != alt_enter
+    assert len(enter) == 1
+    assert len(alt_enter) == 2