MCP connectors v1: admin-registered HTTP (#481)

servers with tools injected across harnesses Adds generic MCP (Model Context Protocol) support. A JSON-RPC 2.0 HTTP client accepts plain-JSON and SSE-framed responses with none, bearer, or OAuth2 client-credentials auth; a durable registry stores admin-registered servers; and a tool service caches tools/list snapshots (refreshing on registry change and on an interval), namespaces tools as serverId_toolName, audits each call, and clamps result size. All harnesses receive MCP tools in the shared tool set, results pass through the external-content screen like any untrusted tool output, and read-only contexts keep only read-only servers' tools. An org-admin-only API registers servers with URL validation, secret redaction on read, and a live tools/list probe before save. Deliberately out of scope: stdio transport, MCP resources/prompts, and user-supplied endpoints (SSRF surface); per-user OAuth is left for a follow-up.

**Deployment notes**

New DurableMap mcp_servers via the existing artifactMap pattern — created idempotently at
boot, no migration.
Entirely opt-in: no tools injected until an admin registers a server via the new /v1/admin/mcp-
servers routes; no new env vars or secrets required at upgrade.
Server auth secrets (bearer/client-credentials) are stored in the durable map, i.e. plaintext in
Postgres, redacted only at the API — same posture as other stored config, but worth a line in docs
for orgs with stricter secret handling.
Blue-green: old instances ignore the table and inject no MCP tools during overlap — sessions
differ in toolset across instances for the deploy window, which the tool-schema path already
tolerates (toolsets vary per turn).

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