fix: correct exit codes, download destination, and file walking

[ayoubfaouzi]

Summary

Third item from the improvement backlog — a batch of small standalone bug fixes:

• Failures now exit non-zero. main() ignored the error returned by cmd.Execute(), so every failure exited with code 0. All commands are converted from Run (+ scattered log.Fatalf) to RunE, errors propagate to main, and the process exits 1. SilenceUsage is set so runtime errors print once instead of being followed by the full usage text. The souk gen/souk add subcommands also no longer silently drop the errors their helpers already returned.
• download -o default fixed. It defaulted to the installed binary's directory (e.g. /usr/local/bin for a packaged install) while the help text claimed the current directory. It now defaults to ., and the flag's help shows the real default.
• scan no longer panics on unreadable paths. The filepath.Walk callback dereferenced its FileInfo without checking the walk error and ignored Walk's return value; it now reuses util.WalkAllFilesInDir, which handles both, and reports an empty walk as an error.
• util.ReadAll short-read fixed. It issued a single Read call, which is not guaranteed to fill the buffer — large files could be silently truncated before hashing/uploading. Replaced with os.ReadFile.
• collectHashes returns an error instead of calling log.Fatalf deep in a helper; new test covers the unreadable-file path.

Test plan

• go build ./... && go vet ./... && go test -race ./... passes locally.
• Manually verified: scan /nonexistent/path prints a single error line and exits 1; download --help shows (default ".").

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