Prepare Branch Context

Create a read-only handoff pack so the next agent can act without re-running basic discovery. Optimize for speed: gather shape first, read only the highest-signal patches, and state unknowns instead of exhaustively proving every edge.

Execution mode: run in a subagent. Default: thrifty unless the user explicitly asks for another agent.

Inputs

• Preferred: prepare-branch-context from <base-branch>
• Also supports: omitted base branch, commits <rev-range>, or files <path1,path2,...>
• If no base is given: detect GitHub default branch; if that fails, fall back to staging and say so.

Fast workflow

1. Resolve scope
   • Determine current branch and BASE_BRANCH.
   • git fetch origin "$BASE_BRANCH"; use origin/$BASE_BRANCH when available, otherwise local $BASE_BRANCH with a note.
   • Respect explicit commit/file subset scopes.
   • If there is no diff from the compare ref, emit a minimal summary and stop.