feat(web): delete a session from the sidebar

A session could be created from the sidebar but never removed, so the
chat surface accumulated every experiment anyone ever named. Each row
now carries a small delete affordance that only names the key: the
confirmation is the shell's, and it says the turn history goes with
the session before calling the new route. On confirm the chat moves to
the first remaining session — the stream's watcher follows the key —
so nothing keeps streaming a session that no longer exists.

The row had to stop being a single button, since a nested button is
invalid HTML: the select button and the delete button now sit in a
container that owns the hover and active surface.

web/claw/app/composables/useClaw.ts

@@ -339,6 +339,11 @@ export interface ClawApi {
   /** `POST /v1/sessions` — create an interactive session from a name;
    * 400 for a name off `SESSION_NAME_PATTERN`, 409 when it exists. */
   createSession(name: string): Promise<NewSession>
+  /** `DELETE /v1/sessions/{key}` — delete a session, its dispatch record
+   * and the turn history recorded under it, so a key re-created under the
+   * same name starts empty. 409 while a turn holds the key, 404 when it is
+   * already gone. */
+  deleteSession(key: string): Promise<void>
   /** `GET /v1/consents` — the pending consents, oldest first. */
   listConsents(): Promise<ConsentRequest[]>
   /** `POST /v1/consents/{id}` — answer one consent; 409 when already
@@ -454,6 +459,13 @@ export function useClaw(token: Ref<string | null>): ClawApi {
     })) as NewSession
   }
 
+  async function deleteSession(key: string): Promise<void> {
+    await send(`/v1/sessions/${encodeURIComponent(key)}`, {
+      method: 'DELETE',
+      headers: authHeaders(token.value),
+    })
+  }
+
   async function listConsents(): Promise<ConsentRequest[]> {
     const body = (await get('/v1/consents', token.value)) as { consents: ConsentRequest[] }
     return body.consents
@@ -722,6 +734,7 @@ export function useClaw(token: Ref<string | null>): ClawApi {
   return {
     sessions,
     createSession,
+    deleteSession,
     listConsents,
     resolveConsent,
     denyConsent,