feat(cli): add ao session set-merge-policy

[Ayush7614]

Summary

• Add ao session set-merge-policy <id> --terminate-on-pr-merge[=true|false]
• Wires existing PATCH /sessions/{id}/merge-policy for scripts/operators
• Requires explicit flag so false is intentional

Test plan

• go test ./internal/cli/ -run TestSessionSetMergePolicy
• Manually toggle true/false and confirm with session get / UI

[somewherelostt]

Thanks for contributing to Agent Orchestrator.

This PR is being picked up by the current external contributor on-call pair:

• @illegalcall
• @Pulkit7070

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