docs(media): keep video off GitHub, document where media lives

Only GIFs that a public page embeds stay in docs/images/: the five README
GIFs plus subagent-demo-20260724.gif, which the published "mission control"
article hotlinks from master. Seven GIFs nothing embeds leave the tree
(their masters are kept in the maintainer's local media library).

.gitignore now blocks *.mp4/*.webm/*.mov/*.m4v/*.mkv/*.avi/*.gif outside
docs/images/*.gif (and the promo branch's Remotion inputs), so a video can
only be committed with an explicit `git add -f`.

docs/media-assets.md records the rule: what is tracked and where it is
embedded, website media (pr/website/assets, deployed separately), release
clips on the design-assets branch, and the local library layout.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -84,6 +84,23 @@ tmp/
 # added there). No trailing slash so it still matches the symlink, not just dirs.
 /public
 
+# Video never goes to GitHub (docs/media-assets.md). The only tracked moving
+# images are the GIFs a public page embeds, in docs/images/; masters, promo cuts,
+# drafts and captures live in the local media library pr/media/ (ignored above).
+# The promo branch tracks its Remotion render inputs, so those stay addable.
+# A deliberate exception elsewhere takes `git add -f`.
+*.mp4
+*.webm
+*.mov
+*.m4v
+*.mkv
+*.avi
+*.gif
+!/docs/images/*.gif
+!/scripts/remotion/public/**/*.mp4
+!/scripts/remotion/public/**/*.webm
+!/scripts/remotion/public/**/*.gif
+
 # Opt-in gesture overlay runtime assets: large MediaPipe wasm + model (~27 MB)
 # fetched at build/install by scripts/fetch-gesture-assets.mjs, kept out of git.
 # (The gesture bundle itself, gesture-codeman.js, IS tracked — built from