Give a nested run-profile child a deterministic agent route

[taras]

Problem

A nested <Execution host="run"> child resolves none of the agent words, so a Markdown test suite cannot drive an agent-using document deterministically. Verified on main (7a4d27ea).

Smallest reproducer, via xmd test:

<Test name="a run child resolves Session">
<Execution host="run" source={'<Session name="s">hi</Session>\n'} as="child">
<AssertEquals actual={child.result.ok} expected={false} />
<AssertStringIncludes actual={child.result.error.message} expected="MARKER" />
</Execution>
</Test>

Failed to import component Session: Cannot resolve component: Session (searched: components, .)
Failed to import component Prompt: Cannot resolve component: Prompt (searched: components, .)

Scope

The gap is in the shared child assembly, not in xmd test itself. runProfileChild (packages/cli/src/testing-host.ts) calls installDocumentComponents({ testing: false }, false) — which installs the agent stack only for mode.testing or mode.agent — and passes executeInstalled(..., []) with no agentIdentityComponents(). The same assembly serves every host of the run profile; in practice it is reached from test documents, because <Execution> is valid only inside a canonical <Test> and <Test> bodies run only under an active testing session. Root documents are unaffected: xmd run has the full live stack, and an xmd test root has the agent words plus <TestAgent>.

Wiring alone is not a fix (measured)

Patching the child to add installAgentComponents() and [{ components: agentIdentityComponents() }] moves the failure exactly one step:

Agent.session() has no provider — install one with installAgentComponents({ rootProvider: { factory, options } })

and every channel a child document could use to supply a provider is runtime-proven dead:

• <AgentProvider name="acpx"> → Unknown agent provider "acpx" — the registry is scope-local and the child's fresh scope holds no registrations.
• <TestAgent> → unresolved (registered only under mode.testing), and it refuses outside an active testing session anyway.
• Providers installed by the enclosing test never reach the child: it runs in a fresh scope (createScope(), packages/testing/src/execution-harness.ts:673), isolation the module header states as design.
• Repository components shadowing Session/Prompt would resolve through the search path, but substituting the trusted orchestration's words is exactly what the Add xmd prompt: turn a request into an approved executable Plan #260 handoff forbids.

What a fix is

A first-class channel that carries scripted agent behavior (and an elicitation provider) across the execution boundary into the child — an architecture feature with a security surface (what may a test hand a child?), not a wiring change. Until then, the deterministic route for agent-using roots is a TypeScript launcher: executeInstalled on the document as root, with agentIdentityComponents() and scripted Agent/Elicitation providers installed around the call.

Why now

The issue #260 xmd prompt prototype's frozen evidence prefers a Markdown suite under packages/cli/tests/document-suites/prompt/; this gap is why that suite cannot exist yet. Full evidence: issue-260-markdown-prototype-implementor-feedback.md (repo root, untracked). Companion contract issue: #640.