fix(sdk,desktop): keep self-targeted p tags on issue assignment and the generic signer

[sumit-m]

Fixes the self-referential p tag strip (#4326) at the point where it keeps
recurring, and covers the case that is visible in the UI.

What is broken today

In Buzz Desktop, open any task in a project and click Assign to me. Nothing
happens. Click Unassign on yourself and nothing happens either — the
operation reports success, a comment appears in the task's history reading
"Assigned this issue to …" / "Unassigned … from this task", and the Assignees
row is unchanged.

Assigning or unassigning anyone else works.

Why

Assignment is modelled as a kind:1 note whose p tags are the assignees.
nostr's EventBuilder::build_with_ctx removes any p tag matching the
author's public key unless allow_self_tagging() was called, so when the
assignee is the signer the published event carries no p tag at all. The
reducer that derives assignees from those notes then adds or removes nobody,
while the human-readable content still names the person.

The event id hashes over the tags that were actually signed, so this is not a
relay-side strip — the tag never existed on the wire.

What this changes

Three call sites, all of which build p tags as payload rather than as
mentions:

• buzz-sdk: build_git_issue_assignee_operation — the assign/unassign
  builder used by the CLI and by Desktop's managed-owner path.
• desktop: build_labeled_recipient_note_event — Desktop's labelled
  recipient notes, which also covers review requests.
• desktop: the sign_event command — the generic signer used from the
  TypeScript side, which is the path the Assignees row actually goes through.

The third is the one worth arguing about, and I think it is the real defect. A
generic signing command's contract is to sign the tags it was given. Silently
discarding one makes it lie to every caller, and each new builder inherits the
trap — which is why this issue keeps being fixed one builder at a time
(#4975 merged for messages and forum notes; #4338, #4484, #3384 open for
membership and PR status). A caller that does not want a self p tag simply
does not add one.

Tests

Two regression tests in buzz-sdk, mirroring the existing
message_preserves_self_mention_p_tag pattern: a self-assignment and a
self-unassignment each keep the assignee's p tag through signing.

Verification

Reproduced and fixed against a local relay: before, clicking Assign to me
published an event with tags ["e", "a", "t", "prior"] and no assignee
appeared; after, the same click keeps the p tag and the name sticks. Existing
events are not repaired — the reducer replays history, and those events
genuinely carry no assignee — so a task assigned before the fix needs
reassigning once.