feat(gbrain-sync): init, restore, uninstall, consumer registry

bin/gstack-brain-init: idempotent first-run. git init ~/.gstack/,
.gitignore=*, canonical .brain-allowlist + .brain-privacy-map.json,
pre-commit secret-scan hook (defense-in-depth), merge driver registration
via git config, gh repo create --private OR arbitrary --remote <url>,
initial push, ~/.gstack-brain-remote.txt for new-machine discovery,
GBrain consumer registration via HTTP POST.

bin/gstack-brain-restore: safe new-machine bootstrap. Refuses clobber
of existing allowlisted files, clones to staging, rsync-copies tracked
files, re-registers merge drivers (required — not cloned from remote),
rehydrates consumers.json, prompts for per-consumer tokens.

bin/gstack-brain-uninstall: clean off-ramp. Removes .git + .brain-*
files + consumers.json + config keys. Preserves user data (learnings,
plans, retros, profile). Optional --delete-remote for GitHub repos.

bin/gstack-brain-consumer + bin/gstack-brain-reader (symlink alias):
registry management. Internal 'consumer' term; user-facing 'reader'
per DX review decision.

bin/gstack-brain-reader

@@ -0,0 +1 @@
+gstack-brain-consumer