fix(desktop): coalesce persona defs with running instances in @ picker (#3947)

[iroiro147]

Summary

Fixes #3947 — the @ mention picker could offer persona-definition rows as managed by you · not in channel while omitting the actual locally managed running instances that are channel members.

Root cause

Two candidate-assembly defects in useMentions.ts:

1. Persona-definition records with empty pubkey were admitted. managed-agents.json can carry persona-definition rows whose pubkey is "" (their running instance carries the real one). addCandidate normalized "" and admitted the row as an agent candidate, so phantom "managed by you" rows rendered and could shadow the real instance — the definition row would never resolve a p tag.

2. Running instances were not enriched with their linked persona's display name. The instance's own name (often null) won, so the picker showed an unnamed instance and the persona row as a separate "managed by you" duplicate instead of coalescing on instance.persona_id == persona.id.

Fix

In desktop/src/features/messages/lib/useMentions.ts:

• addCandidate guard: skip any candidate whose normalized pubkey is blank (if (!pubkey) return;). Persona-definition rows never render as candidates; membership and delivery stay keyed by the instance pubkey.
• Managed-agents loop: when agent.personaId links to an active persona, prefer the persona's displayName for presentation (agent.name || linkedPersonaName || null). The instance row becomes a properly named candidate that coalesces with its channel-member entry instead of rendering as a separate phantom.

Verification

• pnpm typecheck — clean
• pnpm exec biome check src/features/messages/lib/useMentions.ts — clean
• pnpm build:e2e && CI=1 npx playwright test --project=smoke — composer flows exercised

What this does not change

• No change to the persona-definition data model or backend serialization; the empty-pubkey rows are simply ignored at the picker boundary.
• No change to p-tag delivery (membership-keyed path unchanged).
• No fuzzy matching introduced.