refactor: rename AGENT.md to MULTI_AGENT_RULES.md and consolidate INSTRUCTION.md into AGENTS.md

- .agents/AGENT.md -> .agents/MULTI_AGENT_RULES.md
- .agents/AGENT.ko.md -> .agents/MULTI_AGENT_RULES.ko.md
- INSTRUCTION.md -> AGENTS.md (root-level)
- Update create_session.sh onboarding prompt to reference new path
- All cross-references in BOOTSTRAP.md/.ko.md, README.md/.ko.md, SKILL_FEATURES.md updated
- Fix document title headers (# AGENT.md -> # MULTI_AGENT_RULES.md, # CLAUDE.md -> # AGENTS.md)

deploy/install.sh

@@ -128,7 +128,7 @@ if ! check_assets_present "."; then
 
   # Copy non-dev documents if they don't already exist.
   # We skip dev-specific docs like README.md, DONE.md, and FUTURE_WORKS.md.
-  for doc in MESSAGING.md BOOTSTRAP.md BOOTSTRAP.ko.md INSTRUCTION.md; do
+  for doc in MESSAGING.md BOOTSTRAP.md BOOTSTRAP.ko.md AGENTS.md; do
     if [ -f "$STAGE_DIR/$doc" ] && [ ! -e "$doc" ]; then
       cp "$STAGE_DIR/$doc" . || { echo "❌ Error: Failed to copy $doc" >&2; exit 1; }
       echo "$doc" >> "$MANIFEST_FILE"