Always-visible resync / retrigger button for stacks

Workaround

Current workarounds all require elevated access: - `spacectl stack set-current-commit` (requires API key + CLI setup) - GitHub webhook "Redeliver" button (requires GitHub admin/integration access) - Toggling the tracked branch in stack settings (requires stack admin)

Problem

For teams where developers don't have access to the GitHub integration app or spacectl, a missed webhook means they're stuck waiting for someone with admin access to intervene. Request: An always-visible "Resync" or "Retrigger" button on the stack page that checks the VCS for the latest commit and triggers a run if the stack is behind — regardless of whether a webhook was received. Should be available to users with write access to the stack.