Short-form note posts have a defined format and convention

## Summary

Define how a short-form note post reads, where it lives, and how it differs from a longer post in the feeds — the foundation the note syndication and drafting work builds on.

## Motivation

`shape: note` exists in the schema as of #598, so a note is already identifiable. What's still missing is the convention around it: nothing decides how a note carries a page title, where the file lives, or whether it appears in the main feed. `rss.xml` still maps every post to the same title+link item. The full-body syndication and drafting work both need those settled. #191 is the first note and is held pending this.

## Scope

- Resolve the title tension: AstroPaper requires `title`, and a note reads as titleless. Decide how a note carries a page title, and whether that title appears in syndication.
- Decide the note location under `src/data/blog/`. Note that directories are URLs here — `getPath` keeps every non-underscore path segment — so a `notes/` directory would put `/posts/notes/` in the permalink.
- Decide whether notes appear in the main `rss.xml` (title+link) or are excluded in favour of the notes feed (see #322).
- Establish the note format convention an author fills in.

## Acceptance criteria

- [x] A post is unambiguously identifiable as a note — `shape: note`, wired into `content.config.ts` (#598).
- [ ] The note title convention is documented.
- [ ] Main-feed membership for notes is decided and implemented.
- [ ] The convention is documented in `docs/reference/post-shapes.md`.

## Additional context

- Enables #322 (full-body syndication) and #323 (the note-draft skill).
- First note: #191.