Migrate 19 trivial error gates to conductor on_error: (AB#3257)

[PolyphonyRequiem]

Context

docs/on-error-migration-inventory.md documents 19 *_error_gate nodes across 6 workflows, describes all 19 as "trivial" (retry/abort logic with no human judgment call), yet they're surfaced as human gates where operators must click through. This is a direct mission violation: the mission is "human gates for judgment," not "human gates for logistics." AB#3257 defines the acceptance criteria for the migration.

Current impact: operators are interrupted by deterministic retry/abort prompts during every run that encounters a retryable error. Migrating these to conductor on_error: routing removes the interruption and aligns behavior with the mission.

Parent epic: #522 (Squad short-term wins, 2026-05-28 fan-out)

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Acceptance criteria

• All 19 inventoried trivial error gates in docs/on-error-migration-inventory.md migrated to conductor on_error: declarations on the steps that can fail
• Each migrated gate verified against AB#3257 acceptance criteria
• No new human gates introduced as part of this work
• Affected workflows: ado-pr.yaml, github-pr.yaml, actionable.yaml, implement-merge-group.yaml, remedy-stale-descendant.yaml (per inventory)
• On-error migration inventory doc updated to reflect completion

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Owner

• Primary: Wagner (workflow YAML refactoring)
• Advisory: Beethoven (gate placement — verify no judgment gate is accidentally migrated away)

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Estimated effort

1–3 days (inventory-driven mechanical migration; 19 gates across 6 files)

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Reference

• ADO work item: AB#3257
• docs/on-error-migration-inventory.md — full inventory of 19 gates

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File anchors

• docs/on-error-migration-inventory.md
• .conductor/registry/workflows/*.yaml (ado-pr, github-pr, actionable, implement-merge-group, remedy-stale-descendant)

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In-situ clause

Apply DU patterns and hygiene fixes opportunistically where natural in your scope. Don't make these the primary focus.