Align PERFTRACE docs with attribution-branch scope

Drop claims that active-turn nesting and decision tags land in this
slice; those belong to the permission/subagent stack. Keep attribution
docs accurate for what this PR actually ships.

Author: TheGreatAxios

docs/PERFTRACE.md

@@ -36,8 +36,11 @@ turn
 ```
 
 Tags on spans pass through a hard allowlist (`provider_id`, `model_id`,
-`transport`, `decision` allow/deny, token/byte/count numbers, short opaque ids).
-Everything else is stripped at write time — see `src/perf/sanitize.ts`.
+`transport`, token/byte/count numbers, short opaque ids such as `tool_id` and
+`subagent_id`). Everything else is stripped at write time — see
+`src/perf/sanitize.ts`. Permission-wait `decision` tags land with the
+permission/subagent instrumentation stack when that branch is merged.
+
 
 ## Limits and concurrency
 
@@ -47,11 +50,12 @@ Everything else is stripped at write time — see `src/perf/sanitize.ts`.
 | Open spans | `OPEN_SPAN_CAPACITY` (1024) | Oldest **open** span dropped so a leaked `start()` cannot grow without bound |
 | Snapshot | — | Shallow copies of spans/tags; consumers cannot poison the ring |
 
-**Single-primary turn nesting:** `permission.wait` and `subagent` nest under the
-process-wide active turn id (`src/perf/active-turn.ts`). One run-sink reactor
-observer owns that slot. Concurrent multi-observer setups are unsupported —
-later observers overwrite the nesting parent. `clear()`, observer `reset()`, and
-turn close null the slot.
+**Permission / subagent nesting:** `permission.wait` and `subagent` are named
+phases for exclusive attribution when those spans exist. Automatic nesting under
+the open turn is owned by the permission/subagent instrumentation stack
+(CL-5170), not by this attribution slice. Until that stack is on the same
+history, treat those phases as optional categories in rollups when present.
+
 
 ## API surface