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.

scripts/type-equiv.manifest.json

@@ -66,6 +66,11 @@
       "symbol": "SessionEvent",
       "source": "packages/core/session/src/types.ts"
     },
+    {
+      "doc": "docs/core-data-structures/core.md",
+      "symbol": "SendTarget",
+      "source": "packages/core/agent/src/types.ts"
+    },
     {
       "doc": "docs/core-data-structures/core.md",
       "symbol": "SendOptions",
@@ -73,12 +78,22 @@
     },
     {
       "doc": "docs/core-data-structures/core.md",
-      "symbol": "AgentCancelCause",
+      "symbol": "AliasSendOptions",
       "source": "packages/core/agent/src/types.ts"
     },
     {
       "doc": "docs/core-data-structures/core.md",
-      "symbol": "InjectOptions",
+      "symbol": "InboxItemInfo",
+      "source": "packages/core/agent/src/types.ts"
+    },
+    {
+      "doc": "docs/core-data-structures/core.md",
+      "symbol": "CancelOptions",
+      "source": "packages/core/agent/src/types.ts"
+    },
+    {
+      "doc": "docs/core-data-structures/core.md",
+      "symbol": "AgentCancelCause",
       "source": "packages/core/agent/src/types.ts"
     },
     {