Add customizable planning policy with GitHub-native defaults and reusable profiles #1655

Parent epic: #1648

Outcome

Let repositories and downstream products customize planning ontology and policy without forking core Squad workflow logic.

Configure lifecycle gates, hierarchy levels, size policy, required issue sections, milestone policy, issue type/Project/label mappings, label descriptions/colors, required reviews, and activation handoff.

Ship a conservative GitHub-native default plus stricter reusable profiles while keeping downstream execution outside Squad.

Acceptance criteria

• Invalid policy fails visibly before mutation.
• Missing GitHub capabilities use documented fallbacks.
• Defaults never overwrite repository metadata without approval.
• Profiles are composable and versioned.
• Both persistent and ephemeral Squad execution are supported.

Size: L