fix(desktop): preserve cross-install agent identities

[Czaruno]

Summary

• expose retained owner-authored kind:30177 persona links as a non-runnable, community-scoped identity index, then use it in both mention deduplication and the definition Start action before minting a keypair
• make deletion of a relay-synced/bodyless definition local-only so a secondary install cannot publish a kind:5 tombstone for the shared kind:30175
• when a legitimate shared-definition tombstone arrives, snapshot linked local agents into standalone records, preserve their pubkeys/keypairs, and publish the repaired kind:30177 projections
• add an edit-time relink path for already-orphaned agents so they can adopt an active definition without identity rotation
• make tombstone suppression delivery-order independent and exclude deleted retained identities from the new reference index

Closes #3414.
Closes #3440.

Testing

• pnpm test — 3,771 passed
• cargo test --manifest-path desktop/src-tauri/Cargo.toml --lib -- --skip relay_admission::tests — 1,843 passed, 14 ignored; the 13 unrelated relay-admission timing tests were skipped after the unfiltered run stalled in that pre-existing group with all completed tests passing
• targeted identity-link, persona tombstone, retention ordering, detach, and relink regression tests
• cargo clippy --manifest-path desktop/src-tauri/Cargo.toml --lib -- -D warnings
• pnpm typecheck
• pnpm lint
• pnpm check:file-sizes
• pnpm check:px-text
• pnpm check:pubkey-truncation