fix(desktop): preserve agent model/provider when persona snapshot fields are blank

[wpfleger96]

Bug

For agents linked to a persona via "Inherit runtime from persona", the desktop backend re-snapshots the persona onto the agent record at every start and every app-launch restore. The existing code used `persona_snapshot` which unconditionally assigned `snapshot.model` and `snapshot.provider` to the agent record. When the persona has no configured `model`/`provider` (both fields are `Option` and default to `None`), the blank values clobbered whatever model and provider the user had set on the agent. The readiness gate then immediately flagged the agent as needing configuration — a loop that restarted on every app open.

Fix

New helper `persona_field_with_record_fallback` in `persona_events.rs` is the single source of truth for the precedence rule: persona field wins when non-blank (trimmed), otherwise falls back to the record's own field (also blank-normalized). Both-blank stays `None` so a genuinely unconfigured agent still reads as needing setup.

Five call sites updated across three paths:

• Snapshot path (`persona_snapshot_with_agent_config_fallback`): refactored to delegate env-merge/`system_prompt`/`source_version` to `persona_snapshot` and apply the shared helper for `model`/`provider`. Three call sites — agent start (`start_local_agent_with_preflight`), restore backfill (`backfill_persona_snapshots`), and app-launch restore (`restore_managed_agents_on_launch`).
• Deploy path (`build_deploy_payload`): was using `persona.model.or(record.model)` with no blank normalization and dropped `record.provider` entirely when the persona had a blank provider. Provider-backend agents with a blank persona provider now correctly deploy with the record's configured provider.
• Display path (`resolve_effective_prompt_model_provider`): was returning live persona fields with no record fallback, so the config/display surface could show an agent as needing configuration while the spawn path ran it with the record's values. Now matches spawn behavior.

`env_vars` finding

The `persona_snapshot` env-var layering was already correct: persona env is cloned first, then agent overrides are inserted on top — agent keys always win on collision. A persona with an empty env map does not wipe the agent's env vars. No clobber exists in the env path; layering is untouched.

[wesbillman]

Approve — clean, minimal, correct. (Review by Brain, posted via Wes's account.)

• persona_snapshot_with_agent_config_fallback does exactly what the title says: persona wins when non-blank, agent record preserved when persona is blank, both-blank stays None, source_version always refreshed. Whitespace-only correctly treated as blank.
• All three re-snapshot call sites (start preflight, backfill, launch restore) converted; the create-time path correctly keeps plain persona_snapshot — its blank-persona fallback already happens via snapshot_model.or_else(input.model), so no double-fallback.
• The env-var finding in the description checks out in the code: agent overrides merge after persona env, so no clobber existed there.
• 5 unit tests cover every branch incl. whitespace and env layering.
• check-file-sizes.mjs bump is +4 lines on an already-overridden file with justification, consistent with that file's convention.