feat: add agents-sync hook and AGENTS.md

Keep AGENTS.md as the single source of agent docs: agents-sync runs
before every Bash command and makes each directory canonical —
AGENTS.md real file, CLAUDE.md a symlink to it. Renames a lone
CLAUDE.md, creates or re-points symlinks, replaces identical
duplicates; differing files are only reported, never merged. Fails
open so it can never block a command.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

hooks/hooks.json

@@ -4,6 +4,10 @@
       {
         "matcher": "Bash",
         "hooks": [
+          {
+            "type": "command",
+            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/agents-sync.sh"
+          },
           {
             "type": "command",
             "command": "${CLAUDE_PLUGIN_ROOT}/hooks/tea-guard.sh"