Keep secrets in user namespace, fix creds on den teardown

[mkandler]

• Use the established SSH creds for cluster teardown
• Ensures secrets are saved to user's folder instead of default_folder (relevant for organizations)

[sentry-io[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: runhouse/resources/hardware/launcher_utils.py

Function	Unhandled Issue
up	Exception: Received [403] from Den POST 'https://api.run.house/cluster/up': {'detail': 'Not authenticated'} ... Event Count: 3
up	Exception: Received [500] from Den POST 'https://api.run.house/cluster/up': {'code': 404, 'detail': 'No clus... ... Event Count: 2
up	AttributeError: module 'runhouse.rns.rns_client' has no attribute 'username' runhouse.resources.secr... Event Count: 2

📄 File: runhouse/resources/secrets/secret_factory.py (Click to Expand)

| Function | Unhandled Issue | | :------- | :----- | | **`secret`** | [**AttributeError: module 'runhouse.rns.rns_client' has no attribute 'username'**](https://runhouse.sentry.io/issues/6076216709/?referrer=github-open-pr-bot) runhouse.resources.secr...
`Event Count:` **2** |

---

_{Did you find this useful? React with a 👍 or 👎}

[mkandler]

• #1452 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.