Clarify challenge 10 upload target and test with real scp

[madebygps]

Fixes #127

Problem

In #127, scp didn't trigger the challenge 10 flag, while running vi on the VM did. I reproduced this on an Azure VM (Ubuntu 24.04, OpenSSH 9.6):

Scenario	Flag triggered?
scp to ~/file (the command in the issue)	No, wrong directory
scp to ~/ctf_challenges/file	Yes
scp -O (older protocol) to ctf_challenges/	Yes
scp overwriting a file that already exists	No, not a new file
vi in ~/ctf_challenges	Yes, with 4 banners from vim's temporary files

scp itself works. The instructions didn't say that the file must be new and must go into ~/ctf_challenges/. The hint also suggested just creating a file there, which skips the upload.

Changes

• README + verify hint 10: Tell learners to run scp from their own computer into user@ip:~/ctf_challenges/, and that overwriting an existing file doesn't count.
• Monitor (ch10_remote_upload.py): Ignore vim's temporary files (.*.sw?, *~, 4913) and show at most one banner every 5 seconds.
• Test harness: deploy_and_test.sh now triggers challenge 10 with a real scp from the local machine, and test_ctf_challenges.sh just checks that the flag appeared. Before, the test created the file on the VM, so the upload path was never tested.

Not changed

This PR still awards the flag for a file created on the VM itself. We weighed checking that the file really came from an upload (with auditd or a process check) and decided the extra complexity wasn't worth it for this challenge.

Testing

• ./.github/skills/ctf-testing/deploy_and_test.sh azure: PASS, 28 passed and 0 failed; resources destroyed afterwards
• ShellCheck at CI's warning level: clean
• Watcher logic tested locally: temp files give 0 banners, and several files within 5 seconds give 1