feat(dev-setup): render gbrain :user variant to an untracked workspace dir

Stops the dev/Conductor workspace from dirtying tracked SKILL.md source. setup
honors GSTACK_SKIP_GBRAIN_REGEN (passed inline by dev-setup, never exported) and
skips the in-place :user regen; detection is still persisted (PID-unique tmp so
concurrent workspaces can't clobber it). dev-setup instead renders the :user
variant into .claude/gstack-rendered (gitignored, per-workspace) and repoints
the workspace SKILL.md symlinks at it, so the workspace gets brain-aware blocks
while the worktree stays canonical. dev-teardown removes the render.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -7,6 +7,7 @@ make-pdf/dist/
 bin/gstack-global-discover*
 .gstack/
 .claude/skills/
+.claude/gstack-rendered/
 .claude/scheduled_tasks.lock
 .claude/*.lock
 .agents/