Why: worktree feature branches occasionally spawned child branches
(e.g. backend feature/cross-session-reading-1-gcb7f79d) outside the
task-start gate; collaboration.md conditionally allows stacked branches
but the Dev entry never said so, leaving stacking as an automation
blind spot with ad-hoc sha-embedded names.
Evidence: collaboration.md common-baseline section states the stacking
exception verbatim; historical numbered branches are all already merged
into main (origin/main..branch empty); precheck READY at R0=7d45719869.
Impact: AGENTS.md only. Default stays single-level branches from
origin/main with sibling branches for batch subtasks; stacking requires
recording the parent task/PR in the card, child naming without embedded
SHAs, parent-before-child merge order, and a sync back onto latest main
with PR base retargeted before merge (branch protection enforces this).
Refs: task card feature-branch-hierarchy-clause
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