feat: add PR insights analytics dashboard (#23215)

## What

Adds a new admin-only **PR Insights** page for the `/agents` analytics
view — a dashboard for engineering leaders to understand code shipped by
AI agents.

### Backend
- `GET /api/v2/chats/insights/pull-requests` — admin-only endpoint
- 4 SQL queries in `chatinsights.sql` aggregating `chat_diff_statuses`
joined with chat cost data (via root chat tree rollup)
- Runs 5 parallel DB queries: current summary, previous summary (for
trends), time series, per-model breakdown, recent PRs
- SDK types auto-generate to TypeScript

### Frontend (`PRInsightsView`)
- **Stat cards**: PRs created, Merged, Merge rate, Lines shipped,
Cost/merged PR — with trend badges comparing to previous period
- **Activity chart**: Stacked area chart (created/merged/closed) using
git color tokens (`git-added-bright`, `git-merged-bright`,
`git-deleted-bright`)
- **Model performance table**: Per-model PR counts, inline merge rate
bars, diff stats, cost breakdown
- **Recent PRs table**: Status badges, review state icons, author info,
external links
- **Time range filter**: 7d/14d/30d/90d button group
- **4 Storybook stories**: Default, HighPerformance, LowVolume, NoPRs

### Data source
All PR data comes from the existing `chat_diff_statuses` table
(populated by the `gitsync.Worker` background job that polls GitHub
every 120s). No new data collection required.

### Screenshot
View in Storybook: `pages/AgentsPage/PRInsightsView`

coderd/database/dbmock/dbmock.go

@@ -3216,6 +3216,66 @@ func (mr *MockStoreMockRecorder) GetOrganizationsWithPrebuildStatus(ctx, arg any
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetOrganizationsWithPrebuildStatus", reflect.TypeOf((*MockStore)(nil).GetOrganizationsWithPrebuildStatus), ctx, arg)
 }
 
+// GetPRInsightsPerModel mocks base method.
+func (m *MockStore) GetPRInsightsPerModel(ctx context.Context, arg database.GetPRInsightsPerModelParams) ([]database.GetPRInsightsPerModelRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetPRInsightsPerModel", ctx, arg)
+	ret0, _ := ret[0].([]database.GetPRInsightsPerModelRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetPRInsightsPerModel indicates an expected call of GetPRInsightsPerModel.
+func (mr *MockStoreMockRecorder) GetPRInsightsPerModel(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetPRInsightsPerModel", reflect.TypeOf((*MockStore)(nil).GetPRInsightsPerModel), ctx, arg)
+}
+
+// GetPRInsightsRecentPRs mocks base method.
+func (m *MockStore) GetPRInsightsRecentPRs(ctx context.Context, arg database.GetPRInsightsRecentPRsParams) ([]database.GetPRInsightsRecentPRsRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetPRInsightsRecentPRs", ctx, arg)
+	ret0, _ := ret[0].([]database.GetPRInsightsRecentPRsRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetPRInsightsRecentPRs indicates an expected call of GetPRInsightsRecentPRs.
+func (mr *MockStoreMockRecorder) GetPRInsightsRecentPRs(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetPRInsightsRecentPRs", reflect.TypeOf((*MockStore)(nil).GetPRInsightsRecentPRs), ctx, arg)
+}
+
+// GetPRInsightsSummary mocks base method.
+func (m *MockStore) GetPRInsightsSummary(ctx context.Context, arg database.GetPRInsightsSummaryParams) (database.GetPRInsightsSummaryRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetPRInsightsSummary", ctx, arg)
+	ret0, _ := ret[0].(database.GetPRInsightsSummaryRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetPRInsightsSummary indicates an expected call of GetPRInsightsSummary.
+func (mr *MockStoreMockRecorder) GetPRInsightsSummary(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetPRInsightsSummary", reflect.TypeOf((*MockStore)(nil).GetPRInsightsSummary), ctx, arg)
+}
+
+// GetPRInsightsTimeSeries mocks base method.
+func (m *MockStore) GetPRInsightsTimeSeries(ctx context.Context, arg database.GetPRInsightsTimeSeriesParams) ([]database.GetPRInsightsTimeSeriesRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetPRInsightsTimeSeries", ctx, arg)
+	ret0, _ := ret[0].([]database.GetPRInsightsTimeSeriesRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetPRInsightsTimeSeries indicates an expected call of GetPRInsightsTimeSeries.
+func (mr *MockStoreMockRecorder) GetPRInsightsTimeSeries(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetPRInsightsTimeSeries", reflect.TypeOf((*MockStore)(nil).GetPRInsightsTimeSeries), ctx, arg)
+}
+
 // GetParameterSchemasByJobID mocks base method.
 func (m *MockStore) GetParameterSchemasByJobID(ctx context.Context, jobID uuid.UUID) ([]database.ParameterSchema, error) {
 	m.ctrl.T.Helper()