Extract deterministic command steps into bin/ helper scripts

[gringolito]

What

Identify deterministic steps across commands (preflight, milestone resolution, JSON parsing) and extract them into bin/ helper scripts that the commands call.

Why

Reduces prompt drift and duplication; centralizes deterministic logic so commands stay focused on AI reasoning.

In Scope

• Inventory deterministic steps across commands
• Extract them into bin/ scripts
• Rewire the commands to call those scripts

Out of Scope

• Non-deterministic AI reasoning steps

Acceptance Criteria

• Commands call shared bin/ scripts for the extracted deterministic steps

INVEST Notes

Large umbrella. Existing issue #42 (extract shared preflight into bin/backlog-preflight) is the first slice and will be set as a sub-issue of this item.