Let the agent also switch on its own when the active item closes

0.2 previously allowed an autonomous switch only while the active item
waits on the operator. Ready work should not sit behind an idle agent
either, so closing an item (completed or abandoned) is now the second
case. This also defines "ready": the agent can act on the item without
the operator, because it has not started or because intake arrived
while it was parked that answers what it was waiting for. A9's
work_item.switch allows a closed or null `from`, and B2 no longer says
only the operator selects the active project.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/Architecture-Build-Spec.md

@@ -159,13 +159,21 @@ come back to something. Components throughout this document refer to them.
   arrive is a safe point, with one exception: switching is refused while intents
   are unresolved (A2). That keeps every pending intent owned by the active work
   item, so boot reconciliation (A3) stays single-item.
-- **The operator can always switch; the agent may switch on its own in one case.**
+- **The operator can always switch; the agent may switch on its own in two cases,**
-  When the active work item is waiting on the operator (an open gate, a pending
+  when other work items are ready:
-  question, or a halt in `AWAIT_INPUT`) and other work items are ready, the agent
+  - *The active item is waiting on the operator* (an open gate, a pending question,
-  switches to one of them and posts a notice naming both items and why it chose
+    or a halt in `AWAIT_INPUT`). An alert must not sit for hours behind an
-  that one. An alert must not sit for hours behind an unanswered prompt. The agent
+    unanswered prompt. The item it left stays waiting; switching answers nothing
-  never switches away from an item that is working. The item it left stays
+    on its behalf.
-  waiting; switching answers nothing on its behalf.
+  - *The active item has closed* (completed or abandoned). Ready work should not
+    sit behind an idle agent until the operator notices.
+
+  Either way, it posts a notice naming both items and why it chose that one. The
+  agent never switches away from an item that is working.
+- **A parked item is ready when the agent can act on it without the operator:** it
+  has not started yet (a duty finding, a ticket), or intake arrived while it was
+  parked that answers what it was waiting for (an approval, an answer, a review
+  comment). An item still waiting on the operator, with nothing new, is not ready.
 - **The model chooses which ready item to switch to.** This is one of the few
   judgments it is trusted with outright, because choosing grants nothing: every
   item still meets its own gates, and a poor choice costs time, not safety.
@@ -456,7 +464,7 @@ by replaying the operational trail:
   |---|---|
   | `state.transition` | `from`, `to`, `project` |
   | `intent.begin` / `intent.commit` / `intent.abort` | `intent_id`, `kind` |
-  | `work_item.switch` | `from`, `to`, `reason`, `chosen_by` (`model` / `fallback` / `operator`) |
+  | `work_item.switch` | `from` (parked or closed item, or `null`), `to`, `reason`, `chosen_by` (`model` / `fallback` / `operator`) |
   | `gate.open` / `gate.approve` / `gate.reject` | `gate`, `rev`, `channel` |
   | `boot.reconcile` | `action`, per-intent verdicts |
   | `schema.migrate` | `from_version`, `to_version` |
@@ -890,8 +898,9 @@ PR_OPEN
 
 ### B2. Requirements
 
-- **One active project at a time** (park-and-switch, 0.2). Operator selects it;
+- **One active project at a time** (park-and-switch, 0.2). The operator selects it,
-  other projects stay parked at their last position and resume where they left off.
+  or the agent switches on its own under 0.2's rules; other projects stay parked at
+  their last position and resume where they left off.
 - **Operator commands** (over transport): `use <project>`, `approve`,
   `reject <feedback>`, `start milestone <n>`, `status`, `stop` (A4), plus free
   text during planning (forwarded to the agent as answers).