chore: route demo reel GIFs through Git LFS (#1767)

## What
Routes `demo/assets/*.gif` through **Git LFS** and converts the 20
existing reel GIFs to LFS pointers. Adds a Git LFS setup note to
`docs/contributing.md`.

## Why
The demo reel GIFs dominate repo size: the same captures get re-shot and
re-committed, and git keeps every old blob forever (~118 MB of GIF
history backing a ~421 MB `.git`). Routing them through LFS keeps the
binaries out of the packfile so future regenerations stop bloating
history.

## Scope / blast radius
- **Track-forward only** — existing history is left intact. No
`filter-repo` rewrite, no force-push, no re-clone required.
- This does **not** shrink the current `.git` (old blobs remain in
history); it stops the bleeding going forward.
- Contributors must have `git lfs install` configured, or the GIFs
arrive as pointer stubs — documented in `docs/contributing.md`.

## Notes
- GitHub free LFS tier is 1 GB storage + 1 GB/month bandwidth; live GIFs
are ~49 MB. Worth keeping an eye on LFS bandwidth for a trafficked
public repo.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Added Git LFS tracking for GIFs in the demo assets folder to help keep
the repository size manageable.

* **Documentation**
* Updated contributor setup instructions with Git LFS guidance,
including installation, pulling existing large files, and how new GIFs
are handled automatically.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.gitattributes

@@ -0,0 +1,6 @@
+# FNXC:RepoAssets 2026-06-25-09:00:
+# Demo reel GIFs dominate repo size (the same captures get re-shot and re-committed,
+# and git keeps every old blob forever). Route them through Git LFS so the binaries
+# live outside the packfile and future regenerations stop bloating .git.
+# Scoped to demo/assets/*.gif (track-forward only — existing history was left intact).
+demo/assets/*.gif filter=lfs diff=lfs merge=lfs -text

docs/contributing.md

@@ -40,6 +40,19 @@ pnpm build      # default build (excludes desktop/mobile)
 pnpm build:all  # full recursive build including desktop/mobile
 ```
 
+### Git LFS (demo reel GIFs)
+
+The demo reel GIFs in `demo/assets/*.gif` are stored via [Git LFS](https://git-lfs.com) so the binaries live outside the packfile and re-shot captures stop bloating `.git` (see `.gitattributes`).
+
+Install Git LFS **before** cloning or pulling, otherwise these files arrive as small text pointer stubs instead of real GIFs:
+
+```bash
+# macOS: brew install git-lfs   (or your platform's package manager)
+git lfs install        # one-time, per machine
+```
+
+If you already cloned without LFS, run `git lfs install` then `git lfs pull` to hydrate the GIFs. New GIFs added under `demo/assets/` are routed to LFS automatically by the `.gitattributes` filter — no extra steps.
+
 ## Workspace Package Overview
 
 | Package | Purpose |