Fix run-plan Lab stdout aggregate selection

[chubes4]

Summary

• Select the homeboy/agent-task-aggregate/v1 envelope when offloaded agent-task run-plan stdout contains other JSON objects from setup chatter.
• Ignore non-aggregate JSON envelopes during local lifecycle mirroring instead of failing after the remote run has completed.
• Add regression coverage for mixed JSON stdout and non-aggregate stdout.

Verification

• cargo fmt --check
• cargo test run_plan --quiet
• cargo check --quiet
• Full Lab loop site-generation-loop-full-lab-20260609-mirror-guard exited 0 with aggregate totals: 6 succeeded, 0 failed, 0 skipped.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Diagnosed the post-merge Lab mirror parser failure, drafted the aggregate-selection fix and regression tests, and ran the full Lab verification; Chris reviewed the run output and generated PRs.