feat(annotate): carry agent-facing element context on HTML and live-app pinpoints (#1517)

A pinpointed element exported as a one-word placeholder ('[element:
Navigation]') or its flattened textContent. The bridge now captures a
bounded description at click time (tag, id, classes, ancestor path, role,
accessible name, allowlisted attributes, rendered text, an adaptive HTML
skeleton, box, landmark, heading, component hint, live route) as
elementContext; the parent re-validates and re-caps it; the export prints
a fenced skeleton plus selector/path/role/name lines under the comment;
the annotation panel gains a per-row Copy for element-bearing cards; the
feedback archive records element identity. Additive: annotations without
the field export byte-identically, share links drop it, no protocol bump.