ah-9r4 DECISION: worktree/branch topology for agent-created child tasks Past Stand

Blocks the delegation loop. Parent X owns worktree <work_root>/<slug>/task-X on branch agent/task-X. Where does child Y work? git refuses to check the same branch out into two worktrees, so Y cannot enter X's. Branching Y off the default branch means a later merge into X's divergent branch; branching Y off agent/task-X breaks the review/publish path, because gitLogOneline/gitDiffStat (internal/runner/commands.go) compute <default>..HEAD against the repo default branch and would report all of X's work as Y's diff.
Proposed split by whether the role writes code: ADVISORY roles (research/review/design — return text, touch no code) get a detached-HEAD worktree at X's tip. git permits many worktrees on the same COMMIT, just not the same BRANCH, so Y reads X's WIP with no collision and no merge. Only WRITER roles need real branches and a merge story — and the advisory-only subset may cover the useful cases, deferring the writer problem entirely.
Verify the detached-HEAD claim against the pinned git before building on it.

DECISION (verified on git 2.55.0, 2026-07-17):

SPLIT roles by whether they write code.

ADVISORY roles (research/review/design — read parent WIP, return TEXT, publish nothing): each child gets a DETACHED-HEAD worktree at the parent's tip commit:
  git worktree add --detach <child-wt> <parent-tip-sha>
Proven: child sees parent's uncommitted-into-branch WIP (the parent's committed tip), runs on 'HEAD (no branch)', and any commits it makes do NOT move the parent's agent/task-<id> branch. Teardown (git worktree remove) leaves only unreferenced commits — no branch leak, no merge. This is the recommended FIRST and possibly ONLY implementation.

WRITER roles (produce code that must land on the parent's branch): DEFERRED. They need a real branch plus a merge story, and note that gitDiffStat/gitLogOneline compute <default>..HEAD — a writer child branched off agent/task-<id> would misattribute ALL of the parent's work as the child's diff, so a writer child needs a different diff base (merge-base against the parent branch, not the repo default). Do not build until an advisory-only loop proves insufficient.

Confirmed anti-pattern: git refuses 'worktree add <path> agent/task-X' while X's worktree holds that branch ('already used by worktree at ...'), so a child can never simply enter the parent's branch.

CODE SEAM for ah-0ge.1: the runner's worktree creation (internal/runner/commands.go gitWorktreeAddNewBranch, base=default) needs a base-override path for child tasks: detached add at the parent tip instead of a new branch off default. Child worktree path can follow the normal <work_root>/<slug>/task-<childid> convention; only the base differs.

A written decision recording: which roles are advisory vs writer, how each child's worktree base is chosen, and what the diffstat base is for a child (if writer roles are in scope at all).