feat: make download comment idempotent and add --force flag

[jiaxinnns]

Make download refuse to overwrite an existing exercise

Why

gitmastery download <exercise> was silently destructive. If the folder already
existed it printed a warn() — which only echoes, it doesn't prompt or exit — and
then rmtree'd the folder. A student re-running gitmastery d under-control out of
habit lost their work with no consent and no recovery (rmtree takes a backup but
deletes it on success).

For remote + create_fork exercises the damage wasn't only local: the re-download
also closed the student's open PRs and deleted their GitHub fork.

gitmastery progress reset already exists as the sanctioned "start over" path, so
download had no reason to also be one.

What changed

• download now blocks when the target folder exists: it reports that nothing was
  downloaded and points at progress reset (to start over) or --force (to wipe).
  Exits 1. The guard runs before any network work, so it's instant.
• New --force / -f flag opts into the old behaviour. Covers both exercises and
  hands-on practices, and works in the REPL (/download <name> --force).
• The Git/GitHub requirement checks now run before the folder is removed.
  Previously --force deleted first and rolled back by deleting again — so a failed
  check left the student with nothing. The base files are staged in a temp dir, the
  checks run, and only then is the existing folder replaced.
• Fixed a crash: --force over a folder with no valid .gitmastery-exercise.json
  (a partial download, or a folder the student made) raised an unhandled
  FileNotFoundError. Now handled via _read_existing_config.

Before / after

# before                                                                                                
$ gitmastery download under-control
 WARN  You already have under-control, removing it to download again
 ...                                          # work gone, exit 0

# after
$ gitmastery download under-control
 ERROR  You already have under-control downloaded at under-control/. Nothing was downloaded.
        To start the exercise over, run gitmastery progress reset from inside it.
        To delete the folder, along with any work in it, and download it again, run
        gitmastery download under-control --force.
                                              # exit 1, work intact

Note

Behaviour change for anyone scripting a re-download — they'll need --force.
Suggest labelling bump:minor.