docs: report spawn worker delegation scope

docs/report/spawn-worker-delegation-scope-mismatch-20260730.md

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+# SpawnWorker rejects a workspace subdirectory inside the parent write boundary
+
+Date: 2026-07-30
+
+## Symptom
+
+While implementing the multiplexer protocol foundation, the parent Worker could write the checkout but could not delegate `crates/protocol` to a spawned Coder:
+
+```text
+Invalid argument: requested child scope .../checkout/crates/protocol Write is outside this Worker's delegation scope grant
+```
+
+The requested child scope was a recursive subdirectory of the checkout advertised as writable in the parent instructions. No child was created.
+
+## Impact
+
+The parent had to implement and review the protocol slice serially. This removed useful parallel review and made the long Runtime/Server protocol change more interruption-prone.
+
+## Suggested improvement
+
+- Make the effective delegation grant visible separately from the Worker's direct filesystem write scope.
+- Validate and explain which ancestor rule prevents delegation.
+- When a requested child scope is a strict subset of a writable/delegable checkout, accept it consistently.
+- Distinguish “parent may write but may not delegate” from a malformed/out-of-bound scope error.