test(journal): pin bare + agent-prefixed NODE_OPTIONS banner collapse (#1093)

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Summary

Follow-up to #4977 (merged), which landed the strip_recipe_noise
launcher-banner guard in plainify_pr_title but without these additional
regression assertions. This PR lands the remaining test-strengthening so the
issue #1093 anti-weakening coverage is complete in main.

Test-only — no production code change.

What changed

Adds two anti-weakening cases to
plain_summary_drops_node_options_launcher_banner_title in
src/journal/tests_pr_source.rs:

• Bare banner, no Conventional-Commits prefix — the first colon lives in
  the trailing To change: path, so strip_conventional_prefix is a no-op and
  the whole banner must still collapse via strip_recipe_noise.
• Banner still wearing the recipe-runner per-agent log prefix
  ([HH:MM:SS] [amplihack:copilot:PID] …), which strip_recipe_noise peels
  before applying the launcher-line predicate.

Both must collapse to the neutral "A code change." fallback, pinning
currently-correct behaviour against regression.

Test plan

• cargo test --lib journal::tests_pr_source → 14 passed, 0 failed
• Pre-commit gate: cargo fmt --check, cargo clippy --release -D warnings ✅
• Pre-push gate: race-subset release tests + clippy --all-targets --all-features --locked -D warnings ✅

Closes #1093

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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📊 Coverage Summary

Generated by cargo llvm-cov --workspace --summary-only (nightly, excluding test files)

Module	Lines	Covered	Coverage
Total	213367	180286	84.5%

_{Coverage data from CI run. Test files matching tests?/ are excluded from line counts.}