Restructure AGENTS.md into scoped guides

Trim AGENTS.md to whole-repo, every-session facts (154 -> 110 lines) and move
subsystem guidance into docs/guides/*.md, each with paths: frontmatter and a
one-line index entry in AGENTS.md. Symlink each guide from .claude/rules/ so
Claude Code lazy-loads it by matching path; track those symlinks via a
.gitignore exception (.claude/settings.local.json stays ignored).

.gitignore

@@ -10,6 +10,7 @@
 /licensing-service-startos/
 /plans/
 
-# Local machine config / junk
-/.claude/
+# Local machine config / junk — but track the scoped-guide symlinks
+/.claude/*
+!/.claude/rules/
 .DS_Store