Isolate startup-failure test from installed Harnesses

[JacobStephens2]

Summary

Keep the startup-failure test from reaching an installed Harness when PATH lookup skips its non-executable fixture opencode.

 startup-failure child PATH
-  fixture/bin + inherited PATH
+  fixture/bin only (non-executable opencode + real Git symlink)

Add a regression test with a recording opencode on the inherited PATH. Other session-execution fixtures keep their inherited tools.

Closes #652

Evidence

• Before: cargo test startup_failure_never_reaches_an_installed_harness -- --nocapture failed in 0.14s: installed Harness ran.
  After: the regression passes; its recording executable never runs. The existing test still verifies could not run opencode, ErrorKind::PermissionDenied, no completion reporting and no calls file.
• cargo test --bin thirdshift session::execution_tests -- --nocapture: all 10 tests passed. The independent Spec reviewer also ran all 10 successfully.
• cargo clippy --all-targets -- -D warnings: passed.
• cargo fmt --check: passed.
• SYSTEMD_UNIT_PATH=/usr/lib/systemd/system cargo test: full suite passed.

The initial plain cargo test run stopped at site_deploy::the_systemd_units_verify: systemd reported an invalid RequiredBy key in this host's unrelated /etc/systemd/system/career-dashboard-secrets.service. That test passes with SYSTEMD_UNIT_PATH=/usr/lib/systemd/system, using the packaged system units alongside the explicitly supplied repository units.

Seams under test: the isolated session-execution test process, including its PATH lookup, spawn error and completion reporting.

Unaddressed findings

• Standards: none; no findings.
• Spec: none; no findings.

Both review axes read every changed file. Changed files left unread: none.

Merge Danger

Door: two-way

Blast Radius: tests

Only session-execution tests change; rollback is a normal revert.

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