ci: remove outage-era workflow_dispatch from Test Suite

[DecisionNerd]

Summary

Temporary workflow_dispatch on Test Suite bypasses merge-commit semantics of the required CI Gate check (runs on branch head, not refs/pull/N/merge).

Acceptance criteria

• workflow_dispatch removed from .github/workflows/test.yml
• No replacement escape hatch that emits CI Gate on non-merge SHAs

Source

Devinfra Audit Fix-It Plan Wave A / PR2

[coderabbitai]

🔗 Related PRs

#208 - ci: minimize Blacksmith cache storage [merged]
#276 - fix(release): reference M1 publication tracker [merged]
#278 - fix(ci): retain certification evidence as artifacts [merged]
#282 - fix(release): resume immutable v0.5.0 publication [merged]
#427 - build(bazel): cut over CI Gate, retain rollback, retire PR Cargo sticky disks (#4) [merged]

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[DecisionNerd]

Close evidence

Acceptance criteria are met on main via merged PR #435 (7e3bc523e5bb47cf0f9f7d8604df4d2456445900, merged 2026-08-07).

• .github/workflows/test.yml on main has no workflow_dispatch trigger (on: pull_request only).
• The PR body used a broken Closes # reference, so this issue stayed open; closing manually with the merge evidence above.