feat(annotate): All files view in the annotations panel for folder sessions (#1533)

* feat(ui): group annotations by document for multi-document sessions

Pure helpers behind the annotations panel's cross-file view: one group per
document that carries feedback (open document first, the rest by path,
labelled relative to the nearest session root), and the rule that decides
which scope the panel opens on — the saved preference, except that a
document with no feedback of its own while other documents have some opens
on All files.

* feat(ui): All files view in the annotations panel

The panel gains an optional This file | All files scope toggle and a grouped
cross-file timeline built from the same AnnotationCard the single-document
view renders. Cards belonging to another document route their select, edit
and delete to per-document callbacks; useLinkedDoc grows
updateStoredAnnotations so a host can write into a stored document's
annotations (and refuses the active one, whose annotations are host state).

Every new prop is optional: a host that passes none of them renders the
previous panel, legacy '+N in M other files' affordance included.

* feat(annotate): show every document's comments in folder sessions

Wires the panel's All files view into the annotate app: the scope toggle
appears whenever feedback exists outside the open document, the scope is
re-resolved on each document change (and kept across a jump), and cross-file
edits and deletes write into the owning document's store so the export
reflects them. Clicking a card in another document navigates through the file
browser's own selection path, waits for the commit, then selects — which is
what scrolls a markdown highlight or drives the HTML viewer's marker.

Cross-file mutations are deliberately not recorded in the annotation history:
that stack describes the open document's surface.

* docs: cross-file annotations in multi-document annotate sessions

AGENTS.md gets the model (toggle, default rule, grouping, cross-file
mutation and its undo caveat, jump, counts) under Annotation System, and
tests/UI-TESTING.md a nine-step manual checklist.

* chore(guides-show): repin the viewer manifest

The scope toggle's two new Tailwind utilities (.min-w-[16px],
.bg-surface-1/60) moved the viewer's bundled CSS hash. Only the CSS lines
change: the viewer JS hash and every grammar chunk are byte-identical, so
nothing from the annotations panel leaked into the read-only viewer.

* fix(annotate): keep the sidebar closed when a cross-file jump lands on HTML

A jump from the annotations panel follows the same rule as a link click
between HTML documents: an HTML destination owns the viewport and the header
carries its own Back, so arriving there must not pop the left sidebar. The
rule now has one spelling — documentRendersHtml, extracted from
resolveHtmlLinkIntent — which both callers ask.