delete_record returns success for non-existent records

[zzstoatzz]

Summary

delete_record returns a success response when attempting to delete a record that doesn't exist, which can be misleading.

Steps to Reproduce

1. Authenticate as user A (zzstoatzzdevlog.bsky.social / did:plc:o53crari67ge7bvbv273lxln)
2. Attempt to delete a record that belongs to user B (zzstoatzz.io / did:plc:xbtmt2zjwlrfegqvch7fboei) using shorthand URI:

delete_record(uri="tech.waow.slides.deck/3md2ye7te3v2v")

3. Observe the response:

{"deleted": "at://did:plc:o53crari67ge7bvbv273lxln/tech.waow.slides.deck/3md2ye7te3v2v"}

4. The record still exists in the original repo (user B's)

Expected Behavior

One of:

• Return an error or distinct response indicating the record was not found
• Return {"deleted": null, "reason": "not_found"} or similar
• When using shorthand URIs, check if the record exists before claiming success

Actual Behavior

Returns success with the constructed URI (using authenticated user's DID), even though:

• The record never existed in the authenticated user's repo
• The actual record (in user B's repo) is untouched

Root Cause

ATProto's deleteRecord is idempotent - deleting a non-existent record returns success. The MCP constructs the full URI from the authenticated user's DID when given a shorthand, then calls deleteRecord, which "succeeds" on a record that was never there.

Suggested Fix

Before returning success, verify the record existed (either by checking beforehand, or by comparing the state before/after). Alternatively, document this behavior clearly.