feat(agent): unify send(target × wakeup), coalesce context/message into user/message

Replace send/steer/inject with one Agent.send primitive over the
(target × wakeup) matrix; followup/steer/inject become fixed-preset
alias methods on the now-abstract Agent class. Coalesce context/message
into user/message (injected context is a non-user source). Replace
agent/queued with agent/inbox/enqueue/dequeue/discard, add cancel
keepInbox, and add a FIFO-conservation invariant.

packages/core/agent/src/invariant.ts

@@ -24,6 +24,27 @@ const install: InvariantInstaller = (ctx, fail) => {
     }
     lastStatus.set(agent, status)
   }, { global: true })
+
+  // Inbox FIFO conservation: an item leaves the inbox (dequeue) or is dropped
+  // (discard) only after it entered (enqueue), so the live outstanding count
+  // per agent can never go negative. Injection bypasses the FIFOs entirely and
+  // never appears on these events.
+  const outstanding = new WeakMap<Agent, number>()
+  ctx.on('agent/inbox/enqueue', (agent) => {
+    outstanding.set(agent, (outstanding.get(agent) ?? 0) + 1)
+  }, { global: true })
+  ctx.on('agent/inbox/dequeue', (agent) => {
+    const count = outstanding.get(agent) ?? 0
+    if (count <= 0) fail('agent/inbox/dequeue without a matching prior enqueue')
+    outstanding.set(agent, count - 1)
+  }, { global: true })
+  ctx.on('agent/inbox/discard', (agent, items) => {
+    const count = outstanding.get(agent) ?? 0
+    if (items.length > count) {
+      fail(`agent/inbox/discard dropped ${items.length} items but only ${count} were outstanding`)
+    }
+    outstanding.set(agent, count - items.length)
+  }, { global: true })
 }
 
 /**