fix(preset): keep the token meter host-plane and name unjoined agents

Moving the agent plane behind presets left two readers on the wrong side of
the host/agent line.

`dsh-token-meter` was disabled on the host and mounted inside each preset's
`compaction` realm, but its three projection units register into the
process-wide `sessionProjections` table. A unit registered from one preset
answers for every session, so whether a `minimal` session showed a context
meter depended on whether some other session had mounted `standard` since
boot, and a process that only ever ran `minimal` showed none. The meter takes
no configuration, keys every fold by Session, and registers no tool or prompt
section, so it returns to the host composition and leaves the presets'
`isolate` map; the realm and `compact-basic` stay, because what a preset
chooses is whether its agent compacts, not whether its tokens are counted.

Nothing named an agent that joined no preset. The join is a scope-parent link,
and without it the tools, prompt-section, and skill views resolve the empty
global layer: the agent publishes, the turn runs, and the model receives
nothing. `AgentPresets` now logs one warning per such agent while a roster is
configured, and the invariant companion fails outright — at
`system-prompt/assemble` rather than at publication, because an unjoined agent
is legal until it addresses a model and `recompose` binds exactly such an
agent. The warning stays advisory: a synchronous `agent/created` throw vetoes
publication, and the ACP bridge, SDK server, and headless bundle all create an
unjoined agent today.

Three limits are recorded rather than fixed: projection key presence is not a
per-session capability signal, a superseded standing generation is never
reclaimed, and a `cordis_mount` temporary plugin belongs to the composition
rather than the session that mounted it.

Fixes #2203

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

@@ -15,6 +15,9 @@ import { applyChildComposition, childSessionMeta } from '@deepseek-ai/dsh-subage
 import { CallId } from '@deepseek-ai/dsh-llm'
 import type {} from '@deepseek-ai/dsh-skill'
 import type {} from '@deepseek-ai/dsh-tools'
+// Type-only: resolves `ctx.get('sessionProjections')` and `ctx.get('tokenMeter')`.
+import type {} from '@deepseek-ai/dsh-session-projection'
+import type {} from '@deepseek-ai/dsh-token-meter'
 
 const CONFIG_DIR = fileURLToPath(new URL('../config/', import.meta.url))
 const REPO_ROOT = fileURLToPath(new URL('../../..', import.meta.url))
@@ -113,6 +116,31 @@ describe('the shipped Web composition', () => {
     expect(toolNames(ctx)).toEqual([])
   })
 
+  it('keeps the token meter and its context-meter projections on the host plane', async () => {
+    // Read before any preset in this file mounts, which is what makes this an
+    // ownership assertion rather than a mount-order coincidence: a preset-side
+    // meter sits behind an `isolate` realm and is invisible to `ctx.get`.
+    //
+    // The projection registry is process-wide rather than scope-layered, so a
+    // preset-side meter would also make the browser's context meter appear for
+    // a `minimal` session the moment some OTHER session mounted a preset that
+    // carries one, and vanish entirely in a process that only ever ran
+    // `minimal`. Host ownership is what makes the meter a per-session fact.
+    expect(ctx.get('tokenMeter')).toBeDefined()
+    const projections = ctx.get('sessionProjections')
+    if (projections === undefined) throw new Error('the Web composition must compose a projection registry')
+    const handle = await ctx.agents.create({
+      sessionId: SessionId('preset-minimal-meter'),
+      setup: agentCtx => ctx.agentPresets.mount(agentCtx, 'minimal').then(() => undefined),
+    })
+    try {
+      expect(Object.keys(projections.snapshot(handle.agent.session).values).sort())
+        .toEqual(expect.arrayContaining(['contextBreakdown', 'contextPressure', 'tokenUsage']))
+    } finally {
+      await handle.dispose()
+    }
+  })
+
   it('supplies both shipped presets, and only those, from the system root', async () => {
     const listed = await ctx.agentPresets.list()