Fix Ctrl+C handling in CLI - #4411

confirm_ask turned Ctrl+C into sys.exit(1). Callers that catch a second Ctrl+C at their prompt never saw it:

• dstack apply: Ctrl+C at "Stop the run before detaching?" is meant to abort the run. Instead it printed "Cancelled by user", detached and left the run running.
• dstack apply with a preset: Ctrl+C at "Stop creating preset?" is meant to stop it. Instead the agent (started in its own session) kept running along with its runs.

Besides that, Ctrl+C was handled inconsistently. Some commands exited 0 (apply, preset), some exited 1, and the rest printed a traceback.

Changes

• main() handles KeyboardInterrupt once. It prints "Interrupted" to stderr and re-raises, with the traceback hidden by an excepthook. On an uncaught KeyboardInterrupt, Python shuts down normally (including atexit, which closes SSH tunnels) and then kills itself with SIGINT. A calling shell script sees that and stops. Exiting with 130 is not the same: bash then assumes the child handled SIGINT itself and continues the script.
• Removed the local handlers that converted Ctrl+C to an exit code: confirm_ask, dstack login, _repo_diff_verbose (library code, also reachable through the Python API), and the apply and preset catch-alls that exited 0.
• dstack apply: after aborting a run on a second Ctrl+C, re-raise instead of exiting 0.
• Questionary menus (dstack login provider, default project) use unsafe_ask(). ask() swallows Ctrl+C and returns None, so dstack login carried on as if nothing had been selected.
• Fleet provisioning on old servers no longer asks "Delete the fleet before exiting?" with -y, matching the new-server path.

Deliberate stops still exit 0: detaching from a run, the y/n answers at the stop prompts, --watch loops, dstack logs, dstack server (uvicorn swallows the interrupt itself), and the preset commands after their own interrupt handling.

Before/after

Ctrl+C at the dstack login URL prompt, inside for i in 1 2; do dstack login; echo "iteration $i: rc=$?"; done:

Before:

Enter the server URL (https://sky.dstack.ai): ^C
Cancelled by user
iteration 1: rc=1
Enter the server URL (https://sky.dstack.ai): ^C
Cancelled by user
iteration 2: rc=1

After (the loop stops; bash itself exits via SIGINT):

Enter the server URL (https://sky.dstack.ai): ^C
Interrupted

Tests

• The dstack apply and preset double-Ctrl+C tests drive the real confirm_ask and fail without the fix.
• test_main.py covers the handler and the excepthook. It also has one subprocess test (~0.6 s, POSIX only) checking that the process exits via SIGINT without a traceback.

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