Add --path flag to share-link revoke

[AndreyVMarkelov]

Summary

• Add --path flag to share-link revoke to revoke all direct shared links for a Dropbox path
• Lists direct links via ListSharedLinks(Path, DirectOnly=true) with pagination, then revokes each
• --path and URL argument are mutually exclusive
• Returns clear error if no links found for the path
• Rejects root path

Depends on #259.

Test plan

• --path cannot be used with a URL argument
• Empty --path rejected
• Root path rejected
• Lists direct links and revokes all found
• Follows pagination
• Returns error when no links found
• Returns list API errors
• Returns revoke API errors with failing URL context
• Verbose writes status with count to stderr
• Flag registered on command
• golangci-lint clean, all tests pass