Emit harness session handoff packets

[simple-agent-manager]

Summary

• Add packages/harness/handoff with Go structs matching the platform HandoffPacket JSON shape
• Generate a structured handoff at terminal session state via the existing llm.Provider abstraction, with mechanical fallback on LLM failure or malformed JSON
• Persist handoff JSON from the CLI exit path and print its location; derive artifact refs for transcript, modified files, and git branch
• Fix ACP mock-provider final-message updates by only enabling streaming mode when the provider supports streaming

Verification

• cd packages/harness && go test ./...

Coordination

• Target branch: harness/develop
• Do not merge / do not mark ready for self-merge. Leaving open for orchestrator review because parallel harness work is being coordinated.
• Staging deployment verification intentionally skipped: harness-only Go changes; Go tests with mock LLM client are the gate.

[sonarqubecloud]

Quality Gate failed

Failed conditions
B Security Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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