test(ci): isolate custody-test Action summaries from GitHub job summary

[aetherxeg-source]

Context

During an approved external-contributor CI run for PR #128, the GitHub job summary displayed five ExecSurface — ERROR reports and one PASS even though every CI step and Rust test passed. Those are the intentional negative and positive cases of r4_action_custody.rs; they were not failures in the PR under review.

Root cause

r4_action_custody.rs::run_action launches action/run.sh as a child process without overriding the inherited GITHUB_STEP_SUMMARY. The Action intentionally appends its report to that path. During GitHub-hosted cargo test, the six fixture runs therefore append five expected ERROR and one expected PASS sections to the unrelated parent job summary.

Focused change

• Route each fixture's child GITHUB_STEP_SUMMARY to a test-local file.
• Assert that the child still writes a properly headed Action summary.
• Preserve verdict/exit/status and existing negative/positive test semantics.
• Change no production Action, CLI, or policy behavior.

Review gates

• CI on exact PR head: cargo fmt, strict clippy, all Rust tests, lockfile integrity.
• Verify that negative tests still pass and that the GitHub job summary contains no synthetic ExecSurface — ERROR blocks.
• Independent review of scoped diff before merge.

Relevant source: crates/execsurface-cli/tests/r4_action_custody.rs and action/run.sh.

This PR is independent of external contributor PR #128 and does not modify or silently change its evidence or merge qualification.