宜博 / lemongraph

docs: move A/B/D/E/F/G/H's requirements+design docs into docs/done/

Per CLAUDE.md's docs/ role split (todo = pending, done = shipped into the
SDD/code): all seven Graph Engineering series items with a real
implementation (B, D, E, F, G, H -- A had no separate doc, resolved
directly in graphlemon_architecture.md; C is explicitly won't-do) now have
their requirements+design docs moved from docs/todo/ to docs/done/,
git mv'd to preserve history.

Every cross-reference to the 12 moved files -- across the other moved
docs themselves, graph-engineering-open-items.md, 2 Skill files, and 9
src/*.py docstrings/comments -- updated from docs/todo/<file> to
docs/done/<file>. Verified no stale docs/todo/ references to any of the
12 filenames remain anywhere in the repo.

graph-engineering-open-items.md itself stays in docs/todo/ (not moved) --
it's a living index that keeps gaining new entries from later
comparison-note/gap-analysis passes (H itself was added this way, and a
new item already appeared while this cleanup was in progress), not a
single finished conclusion. Its own header now reflects that all of
A/B/D/E/F/G/H are done and explains why it wasn't moved. Also fixed two
pre-existing stray path references (missing todo/done/ prefix entirely,
predating this cleanup) in its G section.

Full src suite (625 tests) passes -- this is a pure documentation/comment
change, no behavior touched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>