Honor optional continuation feedback when stdin is empty

[CBEPX]

Summary

Allow peer-resume-plan --continue to succeed with empty stdin when optional feedback is omitted, while retaining validation for supplied feedback.

Evidence

Four bare peer-resume-plan calls failed. The public form documents --continue [feedback], while handlePeerResumePlan requires JSON stdin and throws on empty input.

Desired outcome

Empty stdin means no feedback; non-empty feedback remains normalized, fenced, and validated.

Acceptance criteria

• Empty stdin produces a valid continuation plan with no feedback.
• Supplied feedback remains normalized and fenced.
• Malformed non-empty input still fails closed.
• Add a focused CLI regression covering the empty and supplied-feedback forms.

Severity/release disposition

High user-facing reliability risk; mandatory v1.7.4 scope.

Provenance

Confirmed by the v1.7.4 reliability audit of peer-resume-plan continuation calls.