fix(desktop): adopt matching folded agent definitions

[brainx]

Summary

• adopt a unique compatible folded definition before the standalone-agent backfill manufactures a new one
• preserve instance-specific runtime, environment, avatar, and behavior settings while rejecting ambiguous or behavior-changing matches
• canonicalize Databricks provider/model data before matching so source hashes reflect final records

Verification

• just ci
• cargo test --manifest-path desktop/src-tauri/Cargo.toml --lib migration::backfill::tests -- --nocapture (18 passed)
• cargo test --manifest-path desktop/src-tauri/Cargo.toml --lib migration::databricks_tests -- --nocapture (11 passed)

Scope

This prevents duplicate definitions when untouched pre-fold stores are upgraded. It deliberately does not garbage-collect or relink stores that already contain both manufactured and folded definitions, because those records may be intentionally distinct.

Addresses #1725

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve record-only model/provider before linking

When a folded definition has a blank/absent model or provider but the standalone record has one, this wildcard match links the record without manufacturing a definition that contains those record-only values. After persona_id is set, agent_event_content omits model/provider for every definition-linked 30177 event (desktop/src-tauri/src/managed_agents/agent_events.rs:70-96), so the next event sync or restore on another device resolves through the incomplete definition and loses the agent's configured model/provider. Require the definition to carry the same non-empty fields, or copy the record values into a definition, before slimming the record.

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