feat(mcp): allow editing stdio MCP sources from the UI

[aryasaatvik]

Problem

The MCP source detail page hides the Edit button for stdio servers because the plugin sets canEdit: sd.transport === "remote" (packages/plugins/mcp/src/sdk/plugin.ts:1382, :1524), and updateSource early-returns on non-remote (plugin.ts:1553). Users can only change command / args / env / cwd by hand-editing executor.jsonc and restarting.

The React side already has a StdioReadOnly view in packages/plugins/mcp/src/react/EditMcpSource.tsx:246 that explains this, but the canEdit gate prevents users from ever reaching it — the button simply doesn't render.

Proposal

Full edit support for stdio sources, parity with remote.

Backend

• Extend McpUpdateSourceInput (plugin.ts:131) with optional command, args, env, cwd.
• Branch updateSource on transport. For stdio:
  • Persist updated McpStdioSourceData.
  • Re-run discovery — tools can change when command / args change — mirroring refreshSource.
  • Replace bindings accordingly.
• Set canEdit: true for stdio at registration (both addSource and refreshSource).
• Extend the API handler and request schema in packages/plugins/mcp/src/api/handlers.ts.

Config-file write-through already handles stdio via toMcpConfigEntry / inputFormFromStored (plugin.ts:1013, :1046), so no work needed there.

UI

• Replace StdioReadOnly in EditMcpSource.tsx:246 with an editable form: command (text), args (string list), env (plain string key/value pairs — no secret references in v1), cwd (text).

Scope notes

• env values are plain strings in v1. Secret references like remote headers use are out of scope.
• Tool re-discovery on save is required: changing the command spawns a different process. Save should re-discover and surface failures inline; stale bindings would be a worse outcome than a failed save.

Open questions

1. Cloud host: stdio is gated by dangerouslyAllowStdioMCP. Should the Edit form be hidden on hosts that disallow stdio? Likely yes — same gate as add.
2. Read-only fallback when stdio is disabled: still surface command / args on the source detail header as info, or keep them hidden entirely?