docs(spec): add top-level README section to relay server design

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/superpowers/specs/2026-05-02-relay-server-design.md

@@ -192,6 +192,24 @@ No integration test against the MCP SSE transport — that is the SDK's responsi
 
 ---
 
+## Top-level README (`docs/superpowers/MULTI-AGENT.md`)
+
+A durable reference document covering the whole development paradigm — not the relay server specifically, but the entire three-terminal workflow that the relay server enables. Lives in `docs/superpowers/` alongside the specs and plans it describes.
+
+**Contents:**
+
+1. **Overview** — the PM/Dev-A/Dev-B pattern: why three terminals, what each role owns, what the user's job is (authorize merges, resolve escalations)
+2. **Starting a lift** — prerequisites checklist, then: `tools/relay/start.sh` → three sessions → paste kickoff prompts
+3. **Coordination protocol reference** — the four message kinds (`status`, `question`, `directive`, `free`), when each is used, what a well-formed body looks like
+4. **Using the relay tools** — `post_message`, `read_messages`, `list_pending` with one-liner examples
+5. **If the relay server isn't running** — fallback to manual copy-paste; the coordination protocol still works, just with the user as bus
+6. **Generating kickoff prompts** — point to the `multi-agent-kickoff` skill; note that the skill injects the relay paragraph automatically
+7. **Ending a lift** — PM emits MERGE-APPROVED, devs push branches, user authorizes merges, Ctrl-C the relay terminal
+
+This README is written for future-you opening the repo six months from now, not for the current lift.
+
+---
+
 ## What this is not
 
 - Not a product feature — never bundled with the extension or CLI