Retry Jupyter proxy requests after token refresh

[jlewi]

Summary

• Retry Jupyter HTTP proxy requests after a 403 when the on-disk server token changed
• Reload the Jupyter server registry from ${RUNME_CONFIG_DIR}/jupyter/*.json before retrying
• Add a regression test that verifies a stale token request is retried with the refreshed token

Root Cause

The Jupyter proxy cached server records in memory and only reloaded them on cache misses. After a local Jupyter server restarted on the same port, the on-disk token changed, but existing cached records kept forwarding the old token and Jupyter returned 403.

Fixes runmedev/web#164

Validation

• go test ./pkg/agent/server
• go test ./pkg/agent/server -run TestForwardHTTP_ReloadsTokenAfterForbidden -count=1
• git diff --check
• runme run lint test passed lint, but full tests are blocked locally by missing integration dependencies (deno, direnv, tree, Dagger Shell digest) and shell-init z noise in PTY tests
• make test-docker was attempted as the documented fallback, but the configured remote Docker context certificate is expired and local Docker Desktop is not running