feat: persist concurrent agents slider at project level

Add `default_concurrency` column to the projects table in the registry
database, allowing each project to remember its preferred concurrency
setting (1-5 agents). The value persists across page refreshes and
app restarts.

Backend changes:
- Add `default_concurrency` column to Project model in registry.py
- Add database migration for existing databases (ALTER TABLE)
- Add get/set_project_concurrency() CRUD functions
- Add ProjectSettingsUpdate schema with validation
- Add PATCH /{name}/settings endpoint in projects router
- Include default_concurrency in ProjectSummary/ProjectDetail responses

Frontend changes:
- Add default_concurrency to ProjectSummary TypeScript interface
- Add ProjectSettingsUpdate type and updateProjectSettings API function
- Add useUpdateProjectSettings React Query mutation hook
- Update AgentControl to accept defaultConcurrency prop
- Sync local state when project changes via useEffect
- Debounce slider changes (500ms) before saving to backend
- Pass defaultConcurrency from selectedProjectData in App.tsx

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/App.tsx

@@ -268,6 +268,7 @@ function App() {
                   <AgentControl
                     projectName={selectedProject}
                     status={wsState.agentStatus}
+                    defaultConcurrency={selectedProjectData?.default_concurrency}
                   />
 
                   <DevServerControl

ui/src/components/AgentControl.tsx

@@ -1,9 +1,10 @@
-import { useState } from 'react'
+import { useState, useEffect, useRef, useCallback } from 'react'
 import { Play, Square, Loader2, GitBranch, Clock } from 'lucide-react'
 import {
   useStartAgent,
   useStopAgent,
   useSettings,
+  useUpdateProjectSettings,
 } from '../hooks/useProjects'
 import { useNextScheduledRun } from '../hooks/useSchedules'
 import { formatNextRun, formatEndTime } from '../lib/timeUtils'
@@ -15,14 +16,47 @@ import { Badge } from '@/components/ui/badge'
 interface AgentControlProps {
   projectName: string
   status: AgentStatus
+  defaultConcurrency?: number
 }
 
-export function AgentControl({ projectName, status }: AgentControlProps) {
+export function AgentControl({ projectName, status, defaultConcurrency = 3 }: AgentControlProps) {
   const { data: settings } = useSettings()
   const yoloMode = settings?.yolo_mode ?? false
 
   // Concurrency: 1 = single agent, 2-5 = parallel
-  const [concurrency, setConcurrency] = useState(3)
+  const [concurrency, setConcurrency] = useState(defaultConcurrency)
+
+  // Sync concurrency when project changes or defaultConcurrency updates
+  useEffect(() => {
+    setConcurrency(defaultConcurrency)
+  }, [defaultConcurrency])
+
+  // Debounced save for concurrency changes
+  const updateProjectSettings = useUpdateProjectSettings(projectName)
+  const saveTimeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null)
+
+  const handleConcurrencyChange = useCallback((newConcurrency: number) => {
+    setConcurrency(newConcurrency)
+
+    // Clear previous timeout
+    if (saveTimeoutRef.current) {
+      clearTimeout(saveTimeoutRef.current)
+    }
+
+    // Debounce save (500ms)
+    saveTimeoutRef.current = setTimeout(() => {
+      updateProjectSettings.mutate({ default_concurrency: newConcurrency })
+    }, 500)
+  }, [updateProjectSettings])
+
+  // Cleanup timeout on unmount
+  useEffect(() => {
+    return () => {
+      if (saveTimeoutRef.current) {
+        clearTimeout(saveTimeoutRef.current)
+      }
+    }
+  }, [])
 
   const startAgent = useStartAgent(projectName)
   const stopAgent = useStopAgent(projectName)
@@ -57,7 +91,7 @@ export function AgentControl({ projectName, status }: AgentControlProps) {
               min={1}
               max={5}
               value={concurrency}
-              onChange={(e) => setConcurrency(Number(e.target.value))}
+              onChange={(e) => handleConcurrencyChange(Number(e.target.value))}
               disabled={isLoading}
               className="w-16 h-2 accent-primary cursor-pointer"
               title={`${concurrency} concurrent agent${concurrency > 1 ? 's' : ''}`}

ui/src/hooks/useProjects.ts

@@ -4,7 +4,7 @@
 
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import * as api from '../lib/api'
-import type { FeatureCreate, FeatureUpdate, ModelsResponse, Settings, SettingsUpdate } from '../lib/types'
+import type { FeatureCreate, FeatureUpdate, ModelsResponse, ProjectSettingsUpdate, Settings, SettingsUpdate } from '../lib/types'
 
 // ============================================================================
 // Projects
@@ -48,6 +48,19 @@ export function useDeleteProject() {
   })
 }
 
+export function useUpdateProjectSettings(projectName: string) {
+  const queryClient = useQueryClient()
+
+  return useMutation({
+    mutationFn: (settings: ProjectSettingsUpdate) =>
+      api.updateProjectSettings(projectName, settings),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['projects'] })
+      queryClient.invalidateQueries({ queryKey: ['project', projectName] })
+    },
+  })
+}
+
 // ============================================================================
 // Features
 // ============================================================================

ui/src/lib/api.ts

@@ -6,6 +6,7 @@ import type {
   ProjectSummary,
   ProjectDetail,
   ProjectPrompts,
+  ProjectSettingsUpdate,
   FeatureListResponse,
   Feature,
   FeatureCreate,
@@ -100,6 +101,16 @@ export async function updateProjectPrompts(
   })
 }
 
+export async function updateProjectSettings(
+  name: string,
+  settings: ProjectSettingsUpdate
+): Promise<ProjectDetail> {
+  return fetchJSON(`/projects/${encodeURIComponent(name)}/settings`, {
+    method: 'PATCH',
+    body: JSON.stringify(settings),
+  })
+}
+
 // ============================================================================
 // Features API
 // ============================================================================

ui/src/lib/types.ts

@@ -15,6 +15,7 @@ export interface ProjectSummary {
   path: string
   has_spec: boolean
   stats: ProjectStats
+  default_concurrency: number
 }
 
 export interface ProjectDetail extends ProjectSummary {
@@ -536,6 +537,10 @@ export interface SettingsUpdate {
   testing_agent_ratio?: number
 }
 
+export interface ProjectSettingsUpdate {
+  default_concurrency?: number
+}
+
 // ============================================================================
 // Schedule Types
 // ============================================================================