feat(desktop): pin persona config to agent record at create

[wpfleger96]

Persona-created agents previously re-read the live persona catalog at every spawn, so editing a persona template silently changed running agents on restart. This makes the agent record the authoritative source for spawn config: the persona's system_prompt, model, provider, and env_vars (plus its content hash) are snapshotted onto ManagedAgentRecord at create time, and the spawn and provider-deploy paths read that snapshot instead of the live catalog.

The result is the semantics Will specified:

• Restart (stop + start) reuses the same record, so the agent stays pinned to the config it was created with.
• Respawn (delete + recreate) re-runs create and rewrites the snapshot, picking up the edited persona.

env_vars are pinned alongside the other fields — without that, a persona credential edit (e.g. ANTHROPIC_API_KEY) would leak into a running agent on restart.

Spawn read-path

The shared resolve_effective_prompt_model_provider body is left intact — its build_managed_agent_summary and ModelPicker callers intentionally read the live persona. Only the spawn_agent_child and build_deploy_payload call sites move to the record snapshot. ModelPicker model-discovery stays live-read; it enumerates available models for the dropdown, not the running agent's config.

Drift indicator

build_managed_agent_summary compares the record's pinned persona_source_version against the linked persona's current persona_content_hash:

• Out of date — the persona was edited since create. The Agents menu shows a marker and prompts the user to delete and respawn.
• Orphaned — the linked persona no longer exists. Never flagged out-of-date, since there is nothing to respawn into.

The summary displays the pinned snapshot (what the agent actually runs), so the badge never contradicts the fields beside it.

Dead-code removal

With the record authoritative, the mem/persona engram and fleet-update subsystem became a self-referential write loop nothing read at spawn. Removed: fleet_update.rs, its launch and persona-save triggers, the engram-at-creation write, and the engram-only helpers in persona_events. The public persona catalog primitives (build_persona_event, persona_from_event, PersonaEventContent) from #939 are untouched — they remain the source the record snapshots from.

Migration

Pre-existing persona agents are backfilled from the live persona once at launch (before restore), logging loudly if the linked persona is gone so an agent never silently spawns from an empty snapshot.

Stack: #939 → this PR