feat(integration): add declarative external connection method

[rekram1-node]

Plugins can now declare an external method the same way they declare key: pure data, no callback.

editor.method.update({
  integrationID: "amazon-bedrock",
  method: {
    id: "aws-profile",
    type: "external",
    label: "AWS profile",
    form: [{ key: "profile", type: "string", options: [...], custom: true }],
  },
})

 key       form → prompt for key → POST /connect/key      → Credential.Key { key, configuration: answer }
+external  form ─────────────────→ POST /connect/external → Credential.External { methodID, metadata: answer }

schema    Integration.ExternalMethod (in the Method union)
core      Integration.connection.external: validates answer against method.form, saves the reference
protocol  POST /api/integration/:id/connect/external  { methodID, answer?, label? }
plugin    effect + promise method types, connect.external
clients   TUI /connect · web/desktop connect dialog · opencode auth login

Keeps controller.ts lint-clean as required by lint:changed, which pulled in small unrelated fixes in that file (typed store generics, shared formatServerError).