fix(desktop): persist Edit Agent provider/model edits for linked personas (#4157)

[iroiro147]

Problem

For linked built-in personas (e.g. Honey, Bumble), the Edit Agent dialog drops provider/model/systemPrompt from the instance update because it defers those fields to the persona definition. Built-in definitions ship with provider and model unset, so the edited values are silently discarded — the agent falls back to global defaults and surfaces as "needs configuration" after restart.

Root cause

AgentInstanceEditDialog.handleSubmit sends provider: undefined and model: undefined whenever linkedPersona != null, deferring to the definition (see lines ~700-710). For built-in personas, the definition has no provider/model set, so the definition-first resolver in resolve_linked (effective_config/mod.rs:74-109) falls back to global defaults.

Fix

1. computeLinkedPersonaProviderModelPatch() (new pure helper in personaRuntimeModel.ts) — returns the subset of {provider, model} that differs from the persona definition, or null when nothing changed. Empty/whitespace local values never propagate; linked-write fires only for linked personas.

2. handleSubmit wiring — after the instance mutation succeeds, calls useUpdatePersonaMutation with the patch (spread into UpdatePersonaInput with the persona's existing displayName and systemPrompt preserved) when linkedPersona != null and the patch is non-null.

Unlinked instances are completely unaffected.

Tests

• agentInstanceLinkedPersonaPatch.test.mjs — 8 tests covering: no-op (values match), whitespace-only (never null out), provider-only/model-only/both-diff patches, built-in-unset-fills-local scenario, trim behavior, and the unlinked guard.
• All 3914 existing desktop webview tests pass.
• cargo check --lib on desktop/src-tauri clean.

Refs #4157