Add .agents/ with live-validate command and cross-agent symlinks

Move .claude/ to .agents/ as the canonical location for agent
commands.  Symlink .claude, .codex, and .gemini to .agents/ so all
three agent platforms discover the same commands.  Add CODEX.md
symlink to AGENTS.md (CLAUDE.md and GEMINI.md already existed).

Force-add .claude symlink despite global gitignore.

Add .agents/commands/live-validate.md: instructions for running a
live batch session to verify fragment ordering and buffer integrity
after rendering pipeline changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timvisher-dd

.claude

@@ -0,0 +1 @@
+.agents