README: document listing and revoking existing share links #564

Need

The README shows how to create a share, but does not explain how its creator can inspect or revoke existing links. Document the implemented sotto shares ls, sotto shares ls --json and sotto shares revoke <token> workflow alongside the share examples.

Verified implementation

Source-reviewed on main f92d1f9a3fb424cceb852d04a4b0d1d96d8ad379 in a clean checkout:

• SharesCommand, list_shares() and revoke_share() already implement these commands.
• The text listing includes the active free-link count and each returned token's class, view count/limit, expiry and revocation metadata.
• JSON is an object with active_free_count and links, rather than a bare array.
• GET /shares returns the creator's latest 100 records as metadata, without ciphertext or fragment material.

This is a documentation gap established by source inspection. No live links were listed or revoked. #475 covers creation prompts and limits; this issue concerns managing links after creation.

Acceptance criteria

• Add a compact list/inspect/revoke example using a synthetic token.
• Explain the distinction between singular share (create) and plural shares (manage), and that management requires login to the configured server.
• Explain how to take the public token from the listing for shares revoke; the argument is not the entire recipient URL or its fragment key.
• Explain that listing returns metadata and cannot reconstruct the full recipient link's decryption fragment. It does not open the link or consume a view.
• Describe the JSON object's top-level fields and the latest-100-record limit without presenting it as an unlimited historical inventory.
• Explain that revocation prevents future retrieval through the link; it cannot undo a recipient's earlier retrieval.
• Check command syntax/output descriptions against the implementation/help and use British English.

No implementation change is needed. Follow CONTRIBUTING.md, including DCO sign-off (git commit -s).