fix(desktop): republish agent identity records when a persona rename propagates

[SeanGearin]

Problem

Part of #2423 (renaming personal agents desynchronises identity).

Renaming an agent definition (persona) propagates the new display name to its
linked agent instances (propagate_persona_name_rename in
desktop/src-tauri/src/commands/personas/mod.rs) and saves
managed-agents.json — but, unlike the instance-rename path
(update_managed_agent), it never re-retains the renamed instances' kind:30177
managed-agent identity records. record.name is part of the published identity
projection (agent_event_content), so after a persona rename:

• managed-agents.json says the NEW name,
• the retained kind:30177 row (retention.db → relay flush loop) still carries
  the OLD name, with the OLD created_at.

The stale identity record stays live on the relay until the next app launch,
when the boot-time reconcile (reconcile_agents_in_dir) finally notices the
content diff and republishes. Until that restart, any surface that resolves
agents from kind:30177 records (second desktop of the same owner, CLI, other
NIP-AP clients) sees the OLD name bound to the agent pubkey while the kind:0
profile already shows the NEW one — the name→identity binding desync described
in #2423, and consistent with the report's observation that repairing state
required "a separate restart".

Fix

• Extract the per-record retain body of the boot reconcile into
  managed_agents::reconcile::retain_agent_record(conn, keys, record) -> Result<bool, String>
  — one shared content-diff + monotonic-created_at-bump engine (returns
  whether a row was rewritten). reconcile_agents_in_dir now calls it per
  record (behavior unchanged; existing reconcile tests still pass).
• commands::agents::retain_managed_agent_pending delegates to the shared
  engine instead of carrying a duplicate implementation (same semantics:
  projection-equality no-op guard, monotonic bump, pending_sync = 1).
• update_persona (Phase 1, still under the store lock, after
  save_managed_agents): call retain_managed_agent_pending for every record
  the rename propagated to — mirroring update_managed_agent. Avatar-only
  edits are deliberately excluded (the avatar is not part of the kind:30177
  projection; retaining would be a guaranteed no-op).

No new events, kinds, or APIs — this uses the existing signed-event retention
and flush pipeline, per CONTRIBUTING's guidance to prefer a signed Nostr event
and the existing ingest path over endpoint-specific JSON APIs.

Out of scope (deliberately)

• Rename → runtime restart is Agent rename does not notify running agents: AGENTS.md update invisible mid-session #1823, fixed by open PR fix: agent rename restart, avatar fallback, migration immutability #2507 (spawn_hash).
• Surfacing kind:0 relay profile-sync failures on rename is PR fix(desktop): surface relay profile-sync failure on agent rename #2302/fix(desktop): surface agent rename relay profile sync failure as a warning toast #2279
  territory (and largely superseded by the merged rollback in fix(desktop): refresh cached channel member names #2258).
• Mention-picker UX (owner/status disambiguation) and channel-membership
  repair for stale identities: TS-side, noted in [Bug] Renaming or re-adding personal agents can desynchronise identity and break @mentions #2423, not touched here.

Test evidence

Two new unit tests in desktop/src-tauri/src/managed_agents/reconcile/tests.rs
(same harness as the existing reconcile tests — tempdir + retention.db + fresh
keys, no AppHandle):

• rename_re_retains_identity_record_with_new_name — retain "Fizz", confirm
  flush, rename to "Spark", re-retain: row keeps the pubkey coordinate,
  carries the new name only, is pending_sync, and its created_at is
  strictly past the retained head (replaceable-event acceptance).
• retain_agent_record_is_noop_when_unchanged — an unchanged projection does
  not rewrite the row and produces zero pending_sync churn.

Ran scoped per CONTRIBUTING build discipline (from desktop/src-tauri):

cargo fmt -p buzz-desktop                                  # applied, clean
cargo clippy -p buzz-desktop --lib --tests -- -D warnings  # exit 0, no warnings
cargo test -p buzz-desktop --lib                           # full lib suite

Full buzz-desktop lib suite: 1562 passed, 0 failed, 13 ignored —
including all 12 managed_agents::reconcile tests (10 pre-existing, all
unmodified in behavior, plus the 2 new regression tests above).

Links

• Issue: [Bug] Renaming or re-adding personal agents can desynchronise identity and break @mentions #2423
• Adjacent (no overlap): PR fix: agent rename restart, avatar fallback, migration immutability #2507 (rename-restart, Agent rename does not notify running agents: AGENTS.md update invisible mid-session #1823), PRs fix(desktop): surface relay profile-sync failure on agent rename #2302/fix(desktop): surface agent rename relay profile sync failure as a warning toast #2279
  (kind:0 sync-failure surfacing), merged fix(desktop): refresh cached channel member names #2258 (instance-rename rollback).