docs: add Windows-Specific Quirks section to hermes-agent skill + keystroke diagnostic

Adds a dedicated '## Windows-Specific Quirks' section to the hermes-agent
skill so Windows pitfalls have one discoverable place to evolve. Inaugural
entries cover:

- Input / keybindings — Alt+Enter intercepted by Windows Terminal,
  Ctrl+Enter as the Windows newline keystroke, mintty/git-bash behavior,
  pointer to scripts/keystroke_diagnostic.py for investigation.
- Config / files — UTF-8 BOM HTTP-400 trap.
- execute_code / sandbox — WinError 10106 SYSTEMROOT root cause +
  _WINDOWS_ESSENTIAL_ENV_VARS fix location.
- Testing / contributing — scripts/run_tests.sh POSIX-venv limitation and
  the system-Python workaround, POSIX-only test skip-guard patterns.
- Path / filesystem — line-ending warnings (cosmetic), forward-slash
  portability.

Collapses the old scattered Windows bullets under 'Platform-specific
issues' into a single pointer at the new dedicated section so there's
only one place to maintain this content.

Also adds the scripts/keystroke_diagnostic.py the skill now references —
a small prompt_toolkit Application that prints the Keys.* identifier and
raw escape bytes for every keystroke. Used to establish the Ctrl+Enter
= c-j fact on Windows Terminal; generally useful for anyone adding a
platform-aware keybinding.

scripts/keystroke_diagnostic.py

@@ -0,0 +1,81 @@
+#!/usr/bin/env python3
+"""Diagnose how prompt_toolkit identifies keystrokes in the current terminal.
+
+Useful when adding a keybinding to Hermes (or any prompt_toolkit app) and you
+need to know what the terminal actually delivers — particularly on Windows,
+where terminals can collapse, intercept, or silently remap key combinations.
+
+Usage:
+    # POSIX
+    python scripts/keystroke_diagnostic.py
+
+    # Windows (PowerShell / git-bash / cmd)
+    python scripts\\keystroke_diagnostic.py
+
+Press the key combinations you care about. Each keystroke prints the
+prompt_toolkit `Keys.*` identifier and the raw escape bytes the terminal
+sent. The last 20 keystrokes stay on screen. Ctrl+Q or Ctrl+C to quit.
+
+Common questions this answers:
+    - Does my terminal distinguish Ctrl+Enter from plain Enter?
+      (On Windows Terminal: yes, Ctrl+Enter → c-j, Enter → c-m.)
+    - Does Alt+Enter reach the app, or does the terminal eat it?
+      (Windows Terminal eats it for fullscreen; mintty may too.)
+    - Does Shift+Enter register as a separate key?
+      (Almost never — most terminals collapse it to Enter.)
+    - What byte sequence does Home/End/PageUp/etc. produce?
+
+Example output for Ctrl+Enter on Windows Terminal + PowerShell:
+    key=<Keys.ControlJ: 'c-j'> data='\\n'
+
+Then in Hermes, bind the newline behaviour to that key:
+    @kb.add('c-j')
+    def handle_ctrl_enter(event):
+        event.current_buffer.insert_text('\\n')
+"""
+from prompt_toolkit import Application
+from prompt_toolkit.key_binding import KeyBindings
+from prompt_toolkit.layout import Layout
+from prompt_toolkit.layout.containers import Window
+from prompt_toolkit.layout.controls import FormattedTextControl
+
+
+_HISTORY: list[str] = []
+
+
+def _header() -> list[str]:
+    return [
+        "Keystroke diagnostic — press keys to see how prompt_toolkit sees them.",
+        "Try: Enter, Ctrl+Enter, Shift+Enter, Alt+Enter, Ctrl+J, Ctrl+M, arrows, Home/End.",
+        "Ctrl+Q or Ctrl+C to quit. Last 20 keystrokes shown.",
+        "",
+    ]
+
+
+def _render_text() -> str:
+    return "\n".join(_header() + _HISTORY[-20:])
+
+
+def main() -> None:
+    kb = KeyBindings()
+
+    @kb.add("<any>")
+    def _on_any(event):  # noqa: ANN001 — prompt_toolkit event type
+        parts = []
+        for kp in event.key_sequence:
+            parts.append(f"key={kp.key!r} data={kp.data!r}")
+        _HISTORY.append(" | ".join(parts))
+        event.app.invalidate()
+
+    @kb.add("c-q")
+    @kb.add("c-c")
+    def _quit(event):  # noqa: ANN001
+        event.app.exit()
+
+    control = FormattedTextControl(text=_render_text)
+    layout = Layout(Window(content=control))
+    Application(layout=layout, key_bindings=kb, full_screen=False).run()
+
+
+if __name__ == "__main__":
+    main()