docs: clean up root markdown files and synchronize with v4.4.0 standards

- Relocate legacy implementation_plan.md to docs/contracts/nats-backplane/implementation_plan.md
- Synchronize root documentation (README, README.ko, BOOTSTRAP, BOOTSTRAP.ko, AGENTS, MESSAGING, IMPROVEMENTS) to latest v4.4.0 standards
- Consolidate IMPROVEMENTS.md to single source of truth (5 open / 31 completed tasks 1:1 matching section headers)
- Archive unanimous PASS review reports from Claude, Grok, Codex, and OpenCode along with Plan Rev.2

AGENTS.md

@@ -73,4 +73,4 @@ Read [MULTI_AGENT_RULES.md](.agents/MULTI_AGENT_RULES.md) (or [Korean version](.
 
 **Normal vs. Orchestration Modes**
 - **Normal Mode**: The orchestrator acts as Main Creator and may edit files directly.
-- **Loop Active Mode (`/multi-agent-mux-loop`)**: Direct file mutation is intercepted by `.agents/hooks.json`. Do not seek workarounds when a tool call is denied — delegate the task immediately via `run_loop.sh`.
+- **Loop Active Mode (`/multi-agent-mux-loop`)**: Direct file mutation is intercepted by `.agents/hooks.json`. The loop runtime is powered by the Python core (`lib_py.loop`, eager-import immutable runtime), and the delegation principle to `run_loop.sh` upon hook denial remains identical. Do not seek workarounds when a tool call is denied — delegate the task immediately via `run_loop.sh`.