feat(agent): add addressable queue operations

docs/core-data-structures/core.md

@@ -430,6 +430,33 @@ type SendTarget = 'next-turn' | 'next-step'
 type InboxPlacement = 'queued' | 'steering'
 ```
 
+`InboxItemId` is a process-local branded string minted for each accepted FIFO occurrence. It is intentionally distinct from `MessageId`: sending the same immutable message twice creates two independently addressable pending items.
+
+```ts type-equiv
+/** One independently addressable accepted occurrence in an agent inbox. */
+interface InboxItem {
+  /** Agent-loop-minted occurrence identity. */
+  readonly id: InboxItemId
+  /** Identified message delivered by the caller. */
+  readonly message: UserMessage
+  /** Acceptance-time FIFO classification. */
+  readonly placement: InboxPlacement
+}
+```
+
+```ts type-equiv
+/** A user-requested mutation of one still-pending inbox item. */
+type InboxAction =
+  | { readonly kind: 'edit'; readonly content: ContentBlock[] }
+  | { readonly kind: 'remove' }
+  | { readonly kind: 'promote' }
+```
+
+```ts type-equiv
+/** Result of applying an inbox action at the synchronous ownership boundary. */
+type InboxActionResult = 'applied' | 'not-found'
+```
+
 ```ts type-equiv
 /**
  * Options for the unified {@link Agent.send} primitive over the
@@ -453,7 +480,7 @@ interface SendOptions {
 }
 ```
 
-The fixed-preset aliases own `target` and `wakeup`; their already identified `UserMessage` carries role, content, and provenance. Its `MessageId` remains stable across that message's `agent/inbox/*` events without being returned by the delivery methods. Injection bypasses the FIFOs and never appears on those events.
+The fixed-preset aliases own `target` and `wakeup`; their already identified `UserMessage` carries role, content, and provenance. Its `MessageId` remains stable when an edit replaces the message content, while the enclosing `InboxItemId` identifies one accepted occurrence across `agent/inbox/enqueue`, `agent/inbox/update`, and its terminal dequeue or discard. Injection bypasses the FIFOs and never appears on those events.
 
 ```ts type-equiv
 /** Options for {@link Agent.cancel}. */
@@ -518,6 +545,18 @@ interface Agent {
    */
   send(message: UserMessage, options: SendOptions): void
 
+  /**
+   * Mutate one still-pending inbox occurrence synchronously. Editing preserves
+   * the message identity and queue position; removal publishes its terminal
+   * discard; promotion moves it to the front of its current FIFO and makes a
+   * queued item waking. A driver-claimed item is no longer pending and returns
+   * `not-found`.
+   * @param id - independently addressable inbox occurrence.
+   * @param action - edit, remove, or promote operation.
+   * @returns whether the pending occurrence was found and updated.
+   */
+  updateInbox(id: InboxItemId, action: InboxAction): InboxActionResult
+
   /**
    * Clear queued and steering work — unless `keepInbox` — and abort the active
    * turn. An effective call first emits `agent/cancel-requested` with the