fix(apiproxy): serve history events and projections from one log position

Review found two defects in the previous commit's ordering fix.

@pku-xht: `historyStateFor` copied the attached session's events, the handler
then awaited `presenterScopeFor`, and only then read the projection baseline
off the still-live Session. An append during that await served events cut at N
beside a baseline folded to N+1 — one response describing two moments. The
same restructure had also moved the baseline read outside the `try`, so a
failing snapshot escaped the structured `internal` error.

Both awaits now happen before the cut: `historySourceFor` resolves which
session serves the read, `presenterScopeFor` ensures the recorded composition,
and `historyCutOf` then reads events and baseline adjacently with nothing
between them. The whole sequence is back inside the try.

The invariant judged any scoped assembly with a chain of one as an unjoined
agent, which rejects a legitimate assembly in a standing preset key (that key
has no parent of its own). It now gates on `context.agent` — a scope-only read
is not an agent and is out of range by construction rather than by a premise
about who else calls `assemble` — and asks the roster's own
`composedPreset()` instead of introspecting chain length. The advisory warning
uses the same relation.

Also from review: the `2026-08-05-per-agent-tool-presentation` note still
described `presentAs` as per-agent, which standing mounts made false and this
branch's own rewording contradicts; the duplicated "process-wide unit table"
argument collapses to the Agent Note with pointers from the five copies; a
dead `.sort()` before `arrayContaining`; and change-history narration in the
tool-cordis README.

packages/scaffold/server/src/server.ts

@@ -216,12 +216,10 @@ export class HarnessSdkServer {
   }
 
   private async createSession(sessionId: string): Promise<SessionRecord> {
-    // No preset composition: the SDK server's compositions keep the
-    // model-facing rows in the host plane, so this agent reads them from the
-    // global registry layer. A deployment that configures `dsh-agent-presets`
-    // has to join one here first — registration visibility inherits only along
-    // the `dsh-scope` parent chain, and an agent created without that link
-    // reads an empty global layer and reaches the model with no tools.
+    // No preset composition: this server's compositions keep the model-facing
+    // rows in the host plane, so this agent reads them from the global layer. A
+    // deployment that configures a roster has to join one here first
+    // (@deepseek-ai/dsh-agent-presets README, "Composing a child agent").
     const handle = await this.ctx.agents.create({
       sessionId: SessionId(sessionId),
       meta: { cwd: this.cwd },