fix: address PR #1802 review round

- listChildren reads the session store via strict ctx.get (property proxy
  is caller-scoped), orders candidates branchlessly, narrows the cold-read
  return type, and pins the cost model and store/registry composition gaps
  with tests; per-file coverage restored
- acp-agent and headless-agent compositions mount session-projection; a
  keyless snapshot pins the descriptor-less diagnostic row
- api-proxy cold spec pins header-origin ownership and the legacy
  descriptor-only opt-out
- design note ships as implemented with its English pairing; companion
  notes and core-data-structures pages synced

.agents/notes/proposed/architecture/2026-07-27-session-projection-and-command-log.md

@@ -56,6 +56,10 @@ declare module 'cordis' {
 - Registration is an effect (disposer with the fiber): an unloaded plugin's key disappears from subsequent responses and the client reads it as capability absence — HMR semantics for free. Duplicate keys throw. Domain plugins register under `ctx.inject(['sessionProjections'], …)` so headless assemblies without the registry stay unaffected.
 - The package owns `./invariant` (every served key has a live registration).
 
+### Shipped consumer: the subagent identity unit
+
+The registry's two read faces already serve a shipped consumer beyond this RFC's wire plan: [subagent list identity via the projection unit](../../implemented/architecture/2026-08-06-subagent-list-identity-projection.md) registers a `subagent` unit — the durable mode/label identity folded last-wins from `subagent/descriptor` — and `SubagentService.listChildren` reads it through `snapshot()` for a live child (the watermark cache, zero log reads) and `restore({}, events, 0)` over one persistence inspection for a cold one. The registry contract is unchanged: no failure channel and no new read face — a unit never throws, an absent value is the signal, and how absence renders is that consumer's decision.
+
 ### Wire: projections block on the history tail page
 
 ```ts ignore-check