Break relay → relayflows dep; relay has zero workflow code

Removes all paths that pulled @relayflows/core (or
@agent-relay/workflow-types as a transitive proxy) into relay:

- Delete @agent-relay/workflow-types package (the compat shim)
- Delete @agent-relay/sdk's ./workflows subpath export and the
  packages/sdk/src/workflows/ shim directory
- Drop @relayflows/core from @agent-relay/sdk dependencies
- Delete workflow-step.ts adapters in github/slack/browser
  primitives (moved to @relayflows/core/integrations/*)
- Drop @agent-relay/workflow-types from primitive package.jsons
- Strip createGitHubStep/createSlackStep from SDK barrel and
  drop the corresponding re-exports
- Remove `relay run` (workflow runner) and `relay workflows`
  (templates) CLI commands — replaced by `relayflows run`
- Delete setup.test.ts (covered the deleted CLI surface)
- Drop browser-primitive's MCP `browser.step.execute` /
  `browser_step_execute` tools (step execution is workflow-side)
- Strip path mappings for workflow-types and workflow-step files
  from SDK tsconfigs
- Drop workflow-types build from root typecheck script
- Update docs to reference @relayflows/core for workflow APIs

Dependency direction is now one-way:
  relayflows -> @agent-relay/sdk
  (no arrow back)

Author: willwashburn

README.md

@@ -58,7 +58,7 @@ serve({ fetch: app.fetch, port: Number(process.env.PORT ?? 3001) });
 ```
 
 For the SDK-side wiring that lets workflow agents use the proxy transparently,
-see [`@agent-relay/sdk/workflows`'s proxy-env
+see [`@relayflows/core`'s proxy-env
 module](../sdk/src/workflows/proxy-env.ts) and the `credentialProxy` field on
 `SwarmConfig`.