feat: page titles must be valid, unique file names on Windows and macOS - new/rename/move refuse, lint reports Unportable Titles, new never overwrites (#155)

Files changed:
- CHANGES.md
- README.md
- VERSION
- instructions/page-lifecycle.md
- instructions/wiki-lint/SKILL.md
- kb/CONTRACT.md
- kb/CONVENTIONS.md
- kb/CONVENTIONS.md.template
- tools/CONTRACT.md
- tools/chemenu/commands/_util.py
- tools/chemenu/commands/lint.py
- tools/chemenu/commands/new_page.py
- tools/chemenu/commands/page_ops.py
- tools/chemenu/lint_core.py
- tools/chemenu/tests/test_lint.py
- tools/chemenu/tests/test_new_page.py
- tools/chemenu/tests/test_page_ops.py
- tools/chemenu/tests/test_titles.py
- tools/chemenu/titles.py

README.md

@@ -322,6 +322,10 @@ Ingest incoming/my-notes.md
 - Use human-readable titles with spaces for files: `Hybrid Search.md`, not kebab-case
 - Use singular for entities: `HA Integration.md` (not `HA Integrations.md`)
 - Use wikilinks matching the file name exactly: `[[Entity Name]]`
+- A title is a file name, so it has to work on Windows and macOS as well: no `< > : " / \ | ? *`,
+  no reserved names such as `CON` or `Index`, no trailing dot, and no second page whose title
+  differs only by case. `wikitool new` and `wikitool rename` refuse such titles, `wikitool lint`
+  reports existing ones, and `kb/CONTRACT.md` § Titles are identifiers has the full rule
 - **Titles follow the subject's own established name, not the wiki's language.** `Act Runner` and
   `GitOps Ownership Model` keep theirs. A title is the only identifier a page has - it also lives
   in every wikilink and citation id pointing at it - so translating one is a rename, never an