feat: doc — warren's vault+render absorbed onto the git-object read path

warren scanned a directory (filepath.WalkDir + os.ReadFile). There is no
checkout here, so Scan walks a git tree through gitx instead and feeds the
FromPages seam: the approved head, a pinned sha and a proposal branch are one
code path with a different revision, and nothing downstream of Archive knows
where its pages came from.

Frontmatter is core's, not warren's. Front embeds core.Frontmatter and adds
only what core deliberately does not model — parent, aliases, planned, and the
ordered key list used for display and search text. Two parsers that disagree
about a document header is a bug that surfaces in the ID registry months later.

Documents key on their frontmatter id, falling back to their path: paths move
and ids do not, and a duplicated id resolves to neither document rather than
letting one win silently, matching what the merge already does with the
approved branch. A header core rejects degrades to "no frontmatter" instead of
failing, because --push-option=skip-validation means such a document can exist
and refusing to render it would turn a typo into an outage.