feat(web): let a blank session switch its agent preset

`agentPreset.select` recomposes one session's agent from a different preset.
It is allowed only while the session is blank — once a turn has run, that
history was produced under the preset's tools and swapping them would strand
logged tool calls, so the attempt answers `agent-preset-locked`.

The agent and the session survive; only the preset subtree is swapped. That
was forced by what the host actually owns: api-proxy discards the `AgentHandle`
it creates, and there is no delete RPC, so neither disposing nor recreating the
session was available. Swapping the subtree is also the better answer — the
session id, its workspace attachment, and its projections all stay put.

`recompose` is unmount-then-mount because two compositions cannot coexist: both
would register the same tool names into one layer. So it resolves the new
preset BEFORE tearing anything down (an unknown id is a no-op) and restores
the previous composition when the new one fails to mount, rather than leaving
the agent with no tools at all. Both paths are pinned by test.

Also restores the English half of the `agentPreset.list` README paragraph,
which was lost before the previous commit — and `verify-translation-pairing
--write` recorded the pair as consistent anyway, because it records whatever
state it finds rather than checking the two sides say the same thing.

packages/preset/agent-presets/src/index.ts

@@ -11,10 +11,11 @@
  */
 
 import { Context, Service } from 'cordis'
+import { scopeOf } from '@deepseek-ai/dsh-scope'
 import z from 'schemastery'
 import { settingsNamespace, type SettingsScope } from '@deepseek-ai/dsh-settings'
 import { discoverPresets } from './discovery.ts'
-import { mountPreset, serviceForAgent } from './mount.ts'
+import { mountPreset, serviceForAgent, unmountPresetFor } from './mount.ts'
 import { UnknownPresetError, type AgentPreset, type Config } from './types.ts'
 
 /** Settings namespace carrying the user's chosen default preset. */
@@ -33,7 +34,8 @@ export const AgentPresetSettingsSchema: z<AgentPresetSettings> = z.object({
 
 export { COMPOSITION_FILE, discoverPresets, scanRoot } from './discovery.ts'
 export {
-  inactiveRows, leakedServices, livePresetMounts, mountPreset, serviceForAgent, type PresetMount,
+  inactiveRows, leakedServices, livePresetMounts, mountPreset, serviceForAgent,
+  unmountPresetFor, type PresetMount,
 } from './mount.ts'
 export { PresetMountError, UnknownPresetError } from './types.ts'
 export type { AgentPreset, Config, PresetRoot, PresetTrust } from './types.ts'
@@ -157,6 +159,47 @@ export class AgentPresets extends Service {
   serviceFor<K extends string & keyof Context>(agent: { ctx: Context }, name: K): Context[K] | undefined {
     return serviceForAgent(this.ctx, agent, name)
   }
+
+  /**
+   * Replace the composition installed for one agent.
+   *
+   * Only valid while the agent has produced nothing: swapping tools mid
+   * conversation would leave logged tool calls the new composition cannot make.
+   * The CALLER owns that check — this method does not read session history.
+   *
+   * The swap is unmount-then-mount because two compositions cannot coexist:
+   * both would register the same tool names into one layer. A failed mount
+   * therefore restores the previous composition rather than leaving the agent
+   * with nothing.
+   * @param agentCtx - the agent's scope context.
+   * @param id - the preset to compose the agent from instead.
+   * @returns the preset now installed.
+   * @throws when the preset is unknown or its composition is unusable; the
+   * previous composition is restored first.
+   */
+  async recompose(agentCtx: Context, id: string): Promise<AgentPreset> {
+    const scope = scopeOf(agentCtx)
+    if (scope === undefined) {
+      throw new Error('agent-presets: refusing to recompose an unscoped context')
+    }
+    // Resolve before tearing anything down, so an unknown id leaves the agent
+    // exactly as it was.
+    const preset = await this.resolve(id)
+    const previous = await unmountPresetFor(scope)
+    try {
+      await mountPreset(agentCtx, preset)
+    } catch (error) {
+      if (previous !== undefined && previous !== preset.id) {
+        await this.mount(agentCtx, previous).catch(() => {
+          // The agent now has no composition, but the switch failure below is
+          // the actionable diagnostic and the restore had the same inputs that
+          // worked a moment ago; reporting its failure instead would hide why.
+        })
+      }
+      throw error
+    }
+    return preset
+  }
 }
 
 export default AgentPresets