docs: align command docs with shipped behavior (#2169)

- multi-{plan,execute,backend,frontend,workflow}.md: add an in-file
  prerequisite note for the external ccg-workflow runtime. README.md already
  warns these commands need codeagent-wrapper and the .ccg prompt tree, but
  users meeting them via the installed slash commands never see the README;
  the commands-core module still installs all five by default
- quality-gate.md: describe what scripts/hooks/quality-gate.js actually does.
  The doc advertised '/quality-gate [path] [--fix] [--strict]' with lint/type
  checks, but the script reads the file path from hook stdin JSON, toggles
  behavior via ECC_QUALITY_GATE_FIX / ECC_QUALITY_GATE_STRICT env vars, and
  runs formatters only (Biome/Prettier, gofmt, ruff format)
- claude-devfleet SKILL.md: add a Setup section pointing at the DevFleet
  server repository (github.com/LEC-AI/claude-devfleet, already disclosed in
  mcp-configs/mcp-servers.json) plus the SECURITY.md port-verification note;
  the skill previously assumed a running instance with no way to obtain one
- regenerate docs/COMMAND-REGISTRY.json for the quality-gate description

docs/COMMAND-REGISTRY.json

@@ -637,8 +637,8 @@
     },
     {
       "command": "quality-gate",
-      "description": "Run the ECC quality pipeline for a file or project scope and report remediation steps.",
-      "type": "general",
+      "description": "Run the ECC formatter quality gate for a single file and report remediation steps.",
+      "type": "testing",
       "primaryAgents": [],
       "allAgents": [],
       "skills": [],
@@ -863,12 +863,12 @@
   "statistics": {
     "byType": {
       "build": 2,
-      "general": 8,
+      "general": 7,
       "orchestration": 6,
       "planning": 2,
       "refactoring": 1,
       "review": 9,
-      "testing": 51
+      "testing": 52
     },
     "topAgents": [
       {