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

commands/multi-execute.md

@@ -6,6 +6,8 @@ description: Execute a multi-model implementation plan while preserving Claude a
 
 Multi-model collaborative execution - Get prototype from plan → Claude refactors and implements → Multi-model audit and delivery.
 
+> **Prerequisite:** Requires the external `ccg-workflow` runtime, which is **not** part of the base ECC install. Initialize it with `npx ccg-workflow` to provision `~/.claude/bin/codeagent-wrapper` and the `~/.claude/.ccg/prompts/*` role files this command depends on. Without that runtime, this command will not run correctly.
+
 $ARGUMENTS
 
 ---