Desktop: stop auto-healing agent kind:0 on start and restore

[tlongwell-block]

Problem

The desktop local record was the authority for an agent's avatar. On every agent start (start_managed_agent) and on app-launch restore (restore_managed_agents), a fire-and-forget task called reconcile_agent_profile, which recomputed the avatar from the local ManagedAgentRecord and republished kind:0 whenever it diverged from the relay.

That healed a self-set avatar back to the device default — the one identity field desktop still overwrote without an explicit user action.

Change

Remove the two non-explicit kind:0 writers:

• the start-path reconcile spawn in start_managed_agent (commands/agents.rs)
• the restore reconcile loop in restore_managed_agents (managed_agents/restore.rs)

Desktop now publishes kind:0 only on explicit user action — agent creation and rename/model-edit save, both of which call sync_managed_agent_profile directly and are untouched.

With both call sites gone, the reconcile chain is orphaned, so it's deleted rather than left as dead code (clippy -D warnings would reject it anyway): reconcile_agent_profile, resolve_legacy_avatar, profile_needs_sync, the ProfileReconcileData struct, the now-unused relay helpers query_agent_profile / AgentProfileInfo, and their unit tests.

Behavior change to note

reconcile_agent_profile also performed a one-shot legacy-avatar backfill for pre-PR-921 records with no stored avatar_url. That backfill was coupled to the unwanted background healer; preserving it would reintroduce an implicit background kind:0 writer — exactly what this removes. A legacy agent now backfills on the next explicit edit-save instead of silently on start.

Scope

• No relay changes. Agent signing keys remain in the keyring.
• This is the agreed quick-fix and a strict subset of the larger relay-first identity work (Lane A): it stops desktop healing now; agent-side self-author + import/export combine land on top later.

Validation

• cargo test --lib (desktop): 718 passed, 0 failed.
• cargo clippy --lib --all-targets -- -D warnings: clean for all touched files. One pre-existing manual-contains lint fires at managed_agents/runtime.rs:67 (unmodified by this PR, surfaced by clippy 1.95.0) — out of scope here.
• cargo build (desktop) compiles; pre-commit/pre-push hooks (fmt, rust/desktop/mobile tests) green.

🤖 Plan validated and implementation reviewed by Max.