feat: add Claude Code plugin with pre-registered Authentik OAuth client

Claude Code's .mcp.json now accepts an `oauth` block with a pre-registered
clientId, so the plugin no longer depends on RFC 7591 Dynamic Client
Registration (still unshipped in Authentik — goauthentik/authentik#8751,
milestoned for 2026.8.0). This lets users install shared-memory as a plugin
instead of running the `claude mcp add --client-id ...` one-liner by hand.

OIDC_CLIENT_ID_MCP is a Public PKCE client, so committing it is safe; no
secret is involved. callbackPort 33418 matches the documented one-liner and
is covered by the loopback redirect regex on the Authentik provider.

Verified: both manifests pass `claude plugin validate`, and a local-path
marketplace install on Claude Code 2.1.220 preserves the oauth block through
to the installed cache. Remote (git-sourced) marketplace install is still
untested — see anthropics/claude-ai-mcp#359.

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

plugin/.mcp.json

@@ -0,0 +1,12 @@
+{
+  "mcpServers": {
+    "shared-memory": {
+      "type": "http",
+      "url": "https://memory.dnspegasus.net/api/mcp",
+      "oauth": {
+        "clientId": "5rkRS3rJhn3Ci9swWkxYMIrZ9OggsjOGy3cIOhYY",
+        "callbackPort": 33418
+      }
+    }
+  }
+}