fix: complete Tasks GA promotion (docs, site) (#20927)

## Summary

Completes the Coder Tasks GA promotion by updating swagger tags and
regenerating API documentation and updating the frontend API structure.

## Related

Follows #20923 and #20921 which promoted Tasks from Beta/Experimental to
GA.

---

🤖 This change was written by Claude Sonnet 4.5 Thinking using
[mux](https://github.com/coder/mux) and reviewed by a human 🏂

docs/reference/api/tasks.md

@@ -1,4 +1,4 @@
-# Experimental
+# Tasks
 
 ## List AI tasks