feat(build): incremental org→html export with style.css sentinel

feat(build): incremental org→html export with style.css sentinel

Re-run of the publish script now skips .org files whose sha256 matches

the cached value in a new .org-export-state file at the repo root,

making re-runs effectively instantaneous when only one or two pages

have changed.

style.css gets a separate "shared sentinel" entry. If it has changed

since the last publish, every .org file is re-exported automatically

— the common case of editing CSS no longer requires remembering

--force.

Pass --force (or --rebuild) to ignore the cache and rebuild

everything. AGENTS.org stays excluded from exports.

Also adds a "Build cache (incremental export)" subsection to

AGENTS.org documenting the new behavior, and gitignores the state

file.