chore: genericize plugin manifests for public release

main is now the shareable artifact: placeholder host and clientId, no
instance-specific hostnames, marketplace renamed to cybercove-labs.

The filled-in manifest for the live instance lives on branch
instance/dnspegasus and is installed with a #ref fragment, which
`claude plugin marketplace add` honors and persists even though it is
absent from --help (verified on Claude Code 2.1.220).

README: plugin install is now path A, documenting both the fork-and-edit
and the #ref branch approaches; remaining paths renumbered B/C/D.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

plugin/.mcp.json

@@ -2,9 +2,9 @@
   "mcpServers": {
     "shared-memory": {
       "type": "http",
-      "url": "https://memory.dnspegasus.net/api/mcp",
+      "url": "https://memory.example.com/api/mcp",
       "oauth": {
-        "clientId": "5rkRS3rJhn3Ci9swWkxYMIrZ9OggsjOGy3cIOhYY",
+        "clientId": "<OIDC_CLIENT_ID_MCP>",
         "callbackPort": 33418
       }
     }