fix: default verify_feedback to empty string in single-step claimStep path

[frankleinadsr]

Problem

Single non-loop steps with {{verify_feedback}} in their input_template fail before first execution when no prior retry has populated that key in the run context.

Observed error:

Step input is not ready: missing required template key(s) verify_feedback

Root Cause

claimStep() has two code paths: one for loop steps and one for single steps. The loop path correctly guards against missing verify_feedback by defaulting it to an empty string before calling findMissingTemplateKeys(). The single-step path did not apply the same guard.

When a single step's input_template contained {{verify_feedback}} and no prior retry had populated the key in the run context, findMissingTemplateKeys() reported it as a missing required key and failStepWithMissingInputs() fired immediately — marking the step and run as failed before any agent ever executed the step.

Fix

Added the same guard to the single-step path (~line 658 of src/installer/step-ops.ts):

// Optional retry feedback should not make first-pass single steps fail template resolution.
if (!context["verify_feedback"]) {
  context["verify_feedback"] = "";
}

The falsy check correctly handles both absent keys and empty-string values without overwriting a legitimately populated verify_feedback from a prior retry cycle.

Test

Added tests/single-step-verify-feedback.test.ts with:

• Integration tests: claimStep() succeeds on first pass for a single step with {{verify_feedback}} and preserves real feedback on retry
• Unit tests: demonstrate the guard logic and that the bug existed without it
• Source code presence check: ensures the guard exists in both the loop and single-step paths

All 168 tests pass.

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