Gate task providers by availability (#2189)

* Gate task providers by availability

Implement provider availability gating documented in docs/task-provider-availability.md.

* Remove task provider availability design doc

* Restore task source after provider availability checks

- Keep saved GitLab/Linear defaults from being lost while provider checks hydrate
- Ignore stale Linear status responses after connect or workspace changes

* fix: address review findings

src/shared/task-providers.test.ts

@@ -1,5 +1,9 @@
 import { describe, expect, it } from 'vitest'
-import { normalizeVisibleTaskProviders, resolveVisibleTaskProvider } from './task-providers'
+import {
+  filterAvailableTaskProviders,
+  normalizeVisibleTaskProviders,
+  resolveVisibleTaskProvider
+} from './task-providers'
 
 describe('task providers', () => {
   it('normalizes provider lists while preserving supported order', () => {
@@ -16,4 +20,22 @@ describe('task providers', () => {
   it('resolves hidden preferred providers to the first visible provider', () => {
     expect(resolveVisibleTaskProvider('github', ['linear'])).toBe('linear')
   })
+
+  it('filters runtime-unavailable providers without changing preference normalization', () => {
+    expect(
+      filterAvailableTaskProviders(['github', 'gitlab', 'linear'], {
+        gitlabInstalled: false,
+        linearConnected: true
+      })
+    ).toEqual(['github', 'linear'])
+  })
+
+  it('falls back to GitHub when every preferred provider is unavailable', () => {
+    expect(
+      filterAvailableTaskProviders(['gitlab', 'linear'], {
+        gitlabInstalled: false,
+        linearConnected: false
+      })
+    ).toEqual(['github'])
+  })
 })

src/shared/task-providers.ts

@@ -24,6 +24,28 @@ export function normalizeVisibleTaskProviders(value: unknown): TaskProvider[] {
   return normalized.length > 0 ? normalized : [...TASK_PROVIDERS]
 }
 
+export type TaskProviderAvailability = {
+  gitlabInstalled: boolean
+  linearConnected: boolean
+}
+
+export function filterAvailableTaskProviders(
+  visibleProviders: readonly TaskProvider[],
+  availability: TaskProviderAvailability
+): TaskProvider[] {
+  const available = visibleProviders.filter((provider) => {
+    if (provider === 'github') {
+      return true
+    }
+    if (provider === 'gitlab') {
+      return availability.gitlabInstalled
+    }
+    return availability.linearConnected
+  })
+
+  return available.length > 0 ? available : ['github']
+}
+
 export function resolveVisibleTaskProvider(
   preferred: TaskProvider | null | undefined,
   visibleProviders: readonly TaskProvider[]