feat: add chat context pinning storage and push trigger (#26385)

Foundation for the Workspace Context Sources RFC (phase 3). The agent
push (#25983) and coderd snapshot storage (#26145) already persist
per-agent context snapshots; this PR lands the **chat-side storage**
plus the **`agentapi` push trigger** that a follow-up will use to read
them. It does **not** touch `chatd` and changes no behavior — nothing
wires an implementation yet.

## What changed

- Adds four nullable columns to `chats` — `context_aggregate_hash`,
`context_dirty_since`, `context_dirty_resources`, and `context_error` —
and rebuilds the `chats_expanded` view.
- Adds three queries — `SetChatContextSnapshot`,
`HydrateAgentChatsContext`, `MarkChatsContextDirtyByAgent` — with
`dbauthz` wrappers and `audit` entries. They are store-interface methods
covered by a Postgres test (`TestChatContextHydration`).
- Adds the `agentapi.ContextDirtyMarker` interface and invokes it inside
the `PushContextState` transaction, publishing collected events only
after commit.

## Intentionally inert

There are **no production callers** of the three queries and **no
implementation** wired for `ContextDirtyMarker`, so the push trigger is
dormant. This is deliberate: the PR is the durable storage/query
foundation only.

The actual integration — the `chatd` implementation that
hydrates/dirties chats and backs a refresh endpoint, consuming the
pinned context in prompt building, the rich SDK types + UI, and retiring
the live per-turn pull — lands as a single follow-up PR. Splitting this
way keeps the schema/query layer reviewable on its own and keeps the
integration whole in one place.

Refs #25983, #26145.

<details>
<summary>Decision log</summary>

- **Columns over a side table.** The four `chats` columns are the
durable model (accepting the one-time `chats_expanded` view/CTE churn).
`last_injected_context` is deliberately left untouched — it is
load-bearing for the live per-turn context pull.
- **Keep `agentapi`, drop `chatd`.** The earlier revision wired the
hydrate/dirty implementation through `chatd` and added a `PUT
/chats/{chat}/context` refresh endpoint. Those were removed so this PR
is pure foundation; `agentapi` defines the trigger + interface (it does
not import `chatd`), and the `chatd` implementation arrives with the
full integration.
- **No new experiment flag.** The columns are dark and unread by prompt
building.
- **Authz.** The new query wrappers authorize chat updates under the
chat RBAC object / `ResourceChat`, consistent with the existing system
chat mutators.

</details>

---

🤖 Generated by Coder Agents on behalf of @kylecarbs.

coderd/agentapi/context_test.go

@@ -88,6 +88,70 @@ func TestPushContextState(t *testing.T) {
 		require.True(t, resp.GetAccepted())
 	})
 
+	t.Run("DirtyMarkerInvokedAfterCommit", func(t *testing.T) {
+		t.Parallel()
+
+		api, dbm := makeAPI(t)
+		marker := &fakeDirtyMarker{}
+		api.DirtyMarker = marker
+		expectInTx(dbm)
+
+		dbm.EXPECT().GetLatestWorkspaceAgentContextSnapshot(gomock.Any(), agentID).
+			Return(database.WorkspaceAgentContextSnapshot{}, errNoRows())
+		dbm.EXPECT().UpsertWorkspaceAgentContextSnapshot(gomock.Any(), gomock.Any()).
+			Return(database.WorkspaceAgentContextSnapshot{}, nil)
+		dbm.EXPECT().UpsertWorkspaceAgentContextResource(gomock.Any(), gomock.Any()).
+			Return(database.WorkspaceAgentContextResource{}, nil).Times(1)
+		dbm.EXPECT().DeleteStaleWorkspaceAgentContextResources(gomock.Any(), gomock.Any()).
+			Return(nil)
+
+		resp, err := api.PushContextState(context.Background(), &agentproto.PushContextStateRequest{
+			Version:       1,
+			AggregateHash: []byte{0xaa, 0xbb},
+			SnapshotError: "watcher degraded",
+			Initial:       true,
+			Resources: []*agentproto.ContextResource{
+				instructionResource("/home/coder/AGENTS.md", "hello"),
+			},
+		})
+		require.NoError(t, err)
+		require.True(t, resp.GetAccepted())
+		// The marker runs inside the push transaction and its returned
+		// callback publishes only after the transaction commits.
+		require.Equal(t, 1, marker.called)
+		require.Equal(t, 1, marker.published)
+		require.Equal(t, agentID, marker.gotAgent)
+		require.Equal(t, []byte{0xaa, 0xbb}, marker.gotHash)
+		require.Equal(t, "watcher degraded", marker.gotErr)
+	})
+
+	t.Run("DirtyMarkerSkippedOnDrop", func(t *testing.T) {
+		t.Parallel()
+
+		api, dbm := makeAPI(t)
+		marker := &fakeDirtyMarker{}
+		api.DirtyMarker = marker
+		expectInTx(dbm)
+
+		// A non-initial push at a version not strictly greater than the
+		// stored one is dropped before any write; hydration and the
+		// dirty fan-out must not run.
+		dbm.EXPECT().GetLatestWorkspaceAgentContextSnapshot(gomock.Any(), agentID).
+			Return(database.WorkspaceAgentContextSnapshot{Version: 5}, nil)
+
+		resp, err := api.PushContextState(context.Background(), &agentproto.PushContextStateRequest{
+			Version:       2,
+			AggregateHash: []byte{0x01},
+			Resources: []*agentproto.ContextResource{
+				instructionResource("/home/coder/AGENTS.md", "hello"),
+			},
+		})
+		require.NoError(t, err)
+		require.False(t, resp.GetAccepted())
+		require.Equal(t, 0, marker.called)
+		require.Equal(t, 0, marker.published)
+	})
+
 	t.Run("RejectsEmptyAndDuplicateSources", func(t *testing.T) {
 		t.Parallel()
 
@@ -598,3 +662,22 @@ func mcpServerResource(source, serverName, description string) *agentproto.Conte
 		},
 	}
 }
+
+// fakeDirtyMarker is a test double for agentapi.ContextDirtyMarker. It records
+// the in-transaction call and counts callback invocations so tests can assert
+// the marker runs inside the push transaction and publishes only after commit.
+type fakeDirtyMarker struct {
+	called    int
+	published int
+	gotAgent  uuid.UUID
+	gotHash   []byte
+	gotErr    string
+}
+
+func (f *fakeDirtyMarker) HydrateAndMarkChatsDirty(_ context.Context, _ database.Store, agentID uuid.UUID, aggregateHash []byte, snapshotError string, _ time.Time) (func(), error) {
+	f.called++
+	f.gotAgent = agentID
+	f.gotHash = aggregateHash
+	f.gotErr = snapshotError
+	return func() { f.published++ }, nil
+}