Strip comments and header metadata on ODT export

Each file is now cleaned before conversion: HTML comments are removed
and everything above the configurable Draft marker (default
"### Rough Draft:") is dropped. A "# Title:" line becomes the chapter
heading and a "# Slug:" line becomes an italic caption beneath it.

- new preprocess module (strip_comments + header parse) with tests
- Chapter gains a slug rendered via a Chapter_20_Caption style, italic
  at both paragraph-style and character-run level
- draft_marker config setting + top-bar field
- export title priority: override > # Title: > first # heading > filename

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

README.md

@@ -66,11 +66,43 @@ Terminal=false
    to write to disk. Drag the `⠿` handles to reorder.
 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 the file's first level-1 heading (`# ...`), or the file
-   name. Titles are saved to `titles.json` and sync with git.
+   blank falls back to a `# Title:` line, then the file's first level-1 heading
+   (`# ...`), then the file name. Titles are saved to `titles.json` and sync with
+   git.
 5. Set the **Export** path and press **Export ODT** to produce the concatenated
-   document. Each file becomes a chapter headed by its (overridden or automatic)
-   title; the source's leading `# heading` is consumed as that title slot.
+   document. Each file becomes a chapter headed by its title, optionally followed
+   by a caption from its `# Slug:` line.
+
+## Header stripping on export
+
+Manuscript files often carry editorial notes and metadata above the prose that
+should not appear in the finished document. On export, each file is cleaned up:
+
+* **HTML comments** (`<!-- ... -->`, including multi-line ones) are removed.
+* **The header block** — everything above the **Draft marker** line (set in the
+  top bar; default `### Rough Draft:`) — is dropped. From that header:
+  * `# Title:` becomes the chapter heading (unless a manual override is set), and
+  * `# Slug:` becomes an italic caption printed beneath the heading.
+
+  Any other header text is discarded. Files with no marker line are exported
+  whole, but stray `# Title:` / `# Slug:` lines are still lifted out. Clear the
+  Draft marker field to disable header splitting entirely.
+
+For example, this source file:
+
+```markdown
+# Title: The Gate
+# Slug: in which the door will not open
+<!-- remember to foreshadow the key -->
+outline: she arrives, she knocks, nothing
+
+### Rough Draft:
+
+The prose that actually gets exported begins here.
+```
+
+exports as a chapter titled **The Gate**, captioned *in which the door will not
+open*, containing only the prose below the marker.
 
 ## Files the app writes