Add markdown formatting hotkeys to the editor

Ctrl/Cmd+B/I/E wrap the selection in bold / italic / inline-code markers,
Ctrl/Cmd+Shift+X in strikethrough, and Ctrl/Cmd+K as a link (with the url
placeholder selected). Pressing the same key on already-wrapped text
removes the markers. Applied to the live selection via the TextEdit
cursor range, with the caret/selection restored around the edit.

Documented in the built-in cheatsheet and the README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -75,7 +75,16 @@ Terminal=false
    it), and the **Contrast** slider to brighten dim editor text (0% = theme
    default; higher pushes the text toward white in dark mode / black in light
    mode). Text always wraps to the pane width, so there is no horizontal
-   scrolling.
+   scrolling. Inline markdown has keyboard shortcuts, applied to the current
+   selection (press again on wrapped text to remove the markers):
+
+   | Shortcut | Effect |
+   | --- | --- |
+   | `Ctrl/Cmd + B` | **bold** (`**…**`) |
+   | `Ctrl/Cmd + I` | *italic* (`*…*`) |
+   | `Ctrl/Cmd + E` | `inline code` |
+   | `Ctrl/Cmd + Shift + X` | ~~strikethrough~~ (`~~…~~`) |
+   | `Ctrl/Cmd + K` | link — `[selection](url)`, with `url` selected to replace |
 4. Set the **Chapter title** for the open file if you want to override the
    auto-derived one (the field shows the automatic title as a hint). Leaving it
    blank falls back to a `# Title:` line in the header, and if there is none, to