feat(update): move a page to its declared parent, refuse another space

[willkg]

Fixes #10. update published the body, title, width, labels and status but ignored space and parent, which left the two coordinates outside L9. Plan: _plans/053_update-moves-pages.md (amended with what implementation and review changed).

Upgrade note: update now moves pages. Every file create wrote carries a parent: line (an id, or null for a top-level page). So the first update after upgrading moves any page that someone moved by hand in Confluence back to the parent its file records, subtree included. That is L9 working as written, and it is how labels already behave, but it is a quiet change for anyone who rearranged a tree in the UI. Run update --dry-run (or diff) first to see which pages would move, and edit parent: in those files to keep the new places. This belongs in the release notes.

Parent.

• An id or a .md path moves the page under that page or folder.
• parent: null (or ~, or an empty value) moves it to the top of the space.
• An absent parent leaves the page where it is.
• A moved page goes last among its new siblings. markfluence does not manage sibling order; reordering is done in Confluence.
• Every move uses the v1 move route. It leaves the page version alone, where a v2 parentId change bumps it, so a move stays out of the moved-page check and the unchanged-body skip. v1 is also the only route that reaches the top of a space: v2 silently ignores a null parentId. The measurements are in docs/confluence/api.md.

Checks before any write. Every read the move needs happens first, so --dry-run reports a move that would fail. The parent must resolve, be a number or a .md path, be a page or folder in the page's space, and not be the page itself or below it. The loop check walks the target's parentId chain, since v2's ancestors route needs a scope nothing else here needs. The move is the first write, and a failed move fails the file.

Space. A space is a refusal, never a move. A page outside the file's space, or outside markfluence.yaml's space: default when the file names none, fails with nothing written. Keys compare without regard to case, which is how Confluence resolves them.

Frontmatter against pages: entry. A blank parent is now a value when the two locations are compared, so parent: null in one against an id in the other is a coordinate disagreement and fails the file.

Output.

• A human line: moved under "Runbooks" (placed last; reorder in Confluence if needed), shown even when a later step fails.
• --json gains moved: null | {from, to} on update results. A new key is compatible since json-output: make adding a key a compatible schema change #200, so schema_version stays 1.

Also:

• diff compares parent: null and the project's space default, to match.
• The ".md parent → page id" code that create and diff each had is now internal/parentref, which update uses too.
• pagemeta.Resolved gained Space(root) and Parent().

Verified end to end against a personal space with the built binary:

• a matching parent skipped the file;
• a dry run previewed a move and wrote nothing;
• moving under a page, to the top, and back under a .md parent each left the page at the same version with the body PUT skipped;
• a loop was refused;
• a space declared in the file, and one from the project default, were each refused;
• diff reported the project-default space.