fix: strip subagent wrapper continuation lines

[AliceLJY]

Summary

• strip [Subagent Context] / [Subagent Task] wrapper lines before memory extraction
• treat the follow-on subagent runtime continuation lines as orchestration metadata, not conversation content
• restore the existing strip-envelope-metadata regression expectations on current master

Testing

• node --test test/strip-envelope-metadata.test.mjs

This is split out as a standalone baseline fix so unrelated PRs do not stay red because of the current subagent-wrapper regression on master.

[AliceLJY]

Self-review passed. Summary:

• Simplifies subagent wrapper stripping from complex single-pass lookbehind regex → two-pass line-by-line approach
• Header lines ([Subagent Context/Task]) stripped entirely, continuation patterns caught independently
• Simpler, more maintainable, easier to extend

1 file, +4/-4. mergeStateStatus: CLEAN.

@rwmjhb ready for your review.

[AliceLJY]

Closing — master already has a more robust implementation via stripLeadingRuntimeWrappers() (line 71 of smart-extractor.ts) that covers all the patterns this PR targeted. No remaining unique changes after rebase.