claude-subagents skill: add guidance on splitting a big batch across parallel subagents

claude-plugins/plugins/base/skills/claude-subagents/SKILL.md

@@ -72,6 +72,39 @@ work while the sub-agent grinds.
   turn it loose on the full batch. A prompt bug replicated across 100
   items is 100 cleanups.
 
+## Split a big independent batch across parallel subagents
+
+One subagent is a sequential worker — a 50-item batch takes roughly 50
+items' worth of wall-clock even though nothing about the recipe forces
+serialization. If the items are independent (each one touches its own
+file/issue/row, nothing depends on another item's result) and the batch
+is big enough that wall-clock matters, chunk it across N subagents
+running at once instead of handing the whole thing to one.
+
+- **Split by natural boundaries**, not an arbitrary item count — one
+  subagent per file, per directory, per module, per label, whatever
+  grouping keeps each worker's slice self-contained. A worker that has
+  to coordinate with another worker mid-task isn't actually independent
+  work; re-scope the split until it is.
+- **Isolate each worker's writes.** For a git-based batch, give each
+  worker its own `git worktree` (own working directory, own branch, same
+  underlying repo — cheap, no full reclone) so N workers editing
+  different files never race on the same working tree or index. For a
+  non-git batch (issue relabeling, API calls), independent items don't
+  need filesystem isolation at all — just launch N in parallel.
+- **Launch all N in the background and don't babysit any single one** —
+  same as the one-subagent case, just N background bash tasks instead of
+  one. Check on them as a batch, not by polling each individually in a
+  loop.
+- **You own the merge.** Once all N report done, review + verify each
+  worker's slice (same "don't trust the self-report blind" rule as
+  below), then combine — for a git-based split, that's you merging N
+  branches (or cherry-picking) into one, not each worker pushing/PRing
+  its own slice.
+
+Skip this for a batch small enough to finish in a couple minutes single-
+threaded — the coordination overhead isn't worth it below that size.
+
 ## Resume for follow-ups
 
 `claude --resume <name> -p "Now run the same procedure over the second batch."`
@@ -99,3 +132,6 @@ anything the sub-agent left ambiguous or exempted for a human call.
   suggests something "isn't possible" or "doesn't exist," confirm it
   before writing that conclusion into the prompt; a wrong assumption
   gets replicated across the whole batch.
+- Handing a big independent batch to one subagent instead of splitting
+  it across several in parallel - a 50-item sequential run burns wall-
+  clock the split-by-worktree approach above would've avoided for free.