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)

README.md

@@ -156,8 +156,8 @@ To ensure communication integrity across public MQTT brokers, the backplane inte
 ```text
 .
 ├── .agents/
-│   ├── AGENT.md                 # Agent roles, snapshottings, and execution charter
-│   ├── AGENT.ko.md              # Agent roles, snapshottings, and execution charter (Korean)
+│   ├── MULTI_AGENT_RULES.md     # Agent roles, snapshottings, and execution charter
+│   ├── MULTI_AGENT_RULES.ko.md  # Agent roles, snapshottings, and execution charter (Korean)
 │   └── skills/                  # Core orchestration shell wrappers & libraries
 │       ├── lib.sh               # Shared orchestration library
 │       ├── multi-agent-mux-create/
@@ -205,6 +205,6 @@ For detailed setup instructions, please consult the **[BOOTSTRAP.md](./BOOTSTRAP
 ## 📝 Guidelines for Collaborating Agents
 
 If you are an AI agent newly onboarded to this project:
-1.  Read **[AGENT.md](.agents/AGENT.md)** to align on development constraints and roles (PM, Worker, Reviewer).
-2.  Adhere to the **Pane Snapshotting Rules** in `AGENT.md` (Section 4) to prevent scrollback data loss during long execution steps.
+1.  Read **[MULTI_AGENT_RULES.md](.agents/MULTI_AGENT_RULES.md)** to align on development constraints and roles (PM, Worker, Reviewer).
+2.  Adhere to the **Pane Snapshotting Rules** in `MULTI_AGENT_RULES.md` (Section 4) to prevent scrollback data loss during long execution steps.
 3.  Never modify core logic without submitting a diff to the reviewer sessions for evaluation.