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.

apps/cli/tests/web-agent-presets.e2e.ts

@@ -159,7 +159,9 @@ describe('the shipped Web composition', () => {
       setup: agentCtx => ctx.agentPresets.mount(agentCtx, 'minimal').then(() => undefined),
     })
     try {
-      expect(Object.keys(projections.snapshot(handle.agent.session).values).sort())
+      // A subset assertion: `tasks`, `goal`, and the rest register into the
+      // same process-wide table, and this is about the meter's three units.
+      expect(Object.keys(projections.snapshot(handle.agent.session).values))
         .toEqual(expect.arrayContaining(['contextBreakdown', 'contextPressure', 'tokenUsage']))
     } finally {
       await handle.dispose()