fix(desktop): address workflow deletions at the workflow's actual owner

[chillerno1]

Fixes #2671.

Problem

Deleting an agent-created workflow from the desktop app appears to succeed, but the workflow keeps running and stays in the list. Since workflows are commonly created by agents, this makes most workflows undeletable from the UI.

Root Cause

delete_workflow builds the NIP-09 a-tag coordinate with the caller's pubkey:

let builder = events::build_workflow_delete(&workflow_id, &current_pubkey_hex(&state)?)?;

kind:30620 is addressable by (author, d-tag). For an agent-created workflow, 30620:<caller>:<workflow-id> names a record that does not exist. The relay accepts the kind:5 (a self-addressed delete always passes validation) and deletes nothing; the UI reports success.

Observed live: a delete of an agent-owned workflow produced ["a","30620:<human-pubkey>:<workflow-uuid>"] while the kind:30620 event is authored by the agent — the coordinate can never match.

Fix

Fetch the workflow's kind:30620 event first (the same #d query update_workflow already uses) and build the coordinate from the event's author. Relay-side validation already authorizes the owning human for agent-authored coordinates via is_agent_owner. Deleting an unknown workflow id now surfaces workflow not found instead of silently publishing an unmatchable delete.

This is the client half of the fix. The relay half is #2242: today the relay executes a-tag deletions scoped by the delete's signer, so a human-signed delete of an agent workflow still no-ops server-side. Without this change, #2242 alone won't help desktop users, because the client addresses a record that doesn't exist. Together they make delete work end to end.

Related but deliberately not included: deleted workflows leaving their kind:30620 event queryable (ghost list entries) is #2390, with a relay-side fix open in #1340.

Validation

• cargo test in desktop/src-tauri: 1617 passed, 0 failed, 14 ignored.
• cargo clippy --all-targets and cargo fmt --check: clean.
• Live repro against the hosted relay: the stock client's kind:5 for an agent-owned workflow named the signer in the coordinate; the relay returned accepted: true and the workflow remained live and kept firing.
• With the fix: the delete event carries 30620:<agent-pubkey>:<workflow-id>; an owner-signed deletion of that exact coordinate was verified to remove the workflow record.

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