feat: add Linear integration (#736)

* feat: add Linear integration

Mirrors the existing Jira integration so a workspace can connect to Linear
with a personal API key, browse and search issues, and start a Kandev task
from any issue. Backend uses Linear's GraphQL API with a workspace-scoped
config + secret store and a 90s auth-health poller; the frontend adds a
settings page, an issue dialog with state transitions, link/import buttons
in the task UI, and a paginated /linear browse page with a Start task flow.

* fix(linear): address PR review feedback

- Route httpListStates through Service.ListStates instead of clientFor
- Extract repeated workspace_id required string into a constant
- Log GetConfig errors during credential resolution instead of dropping them
- Clear popover error state on close in import + link buttons
- Pick the first workflow with steps in the quick task launcher (don't hard-pin to workflows[0])
- Reset useLinearEnabled state on workspace change via tag-based pattern
- Add cursor-pointer to anchor + Link elements per project convention
- Add tests: handlers_test.go, linear-api.test.ts, use-linear-enabled.test.ts, linear-issue-common.test.tsx
- Document internal/jira and internal/linear in CLAUDE.md/AGENTS.md

* fix(linear): address second-round PR review feedback

- Round Linear's fractional state position instead of truncating it via
  int(math.Round(s.Position)) so adjacent fractional positions don't collapse
- Force LinearSettings to remount on workspace change with key={id} so the
  previous workspace's config / form / testResult / teams cannot bleed into
  the next workspace before the new fetches settle
- Broaden the legacy-value useLinearEnabled test to cover "true", "1", "yes",
  and "legacy" so a regression to strict value === "true" would fail

* fix(linear): address third-round PR review feedback

- Propagate context.Canceled / context.DeadlineExceeded from ListStates in
  GetIssue instead of swallowing them — a caller cancellation must not return
  a partial-success response
- Drop the one-off contains() test helper in handlers_test.go and use
  strings.Contains directly so the assertion is self-documenting
- Add getLinearIssue URL-shape test that locks identifier-as-path-segment
  vs workspaceId-as-query-param so accidentally swapping them would fail

* test(linear): extract repeated literals in linear-api.test.ts

Pulls "ws-1", the API base URL, and the auth-method literal into module-
level constants so the assertions read as the route shape they care about
and the duplicate-strings ESLint rule (currently warn, threshold 4)
stays quiet as the file grows.

---------

Co-authored-by: Kandev Agent <agent@kandev.dev>

apps/web/components/kanban/kanban-header.tsx

@@ -15,8 +15,10 @@ import {
   IconTimeline,
   IconBrandGithub,
   IconTicket,
+  IconHexagon,
 } from "@tabler/icons-react";
 import { useJiraAvailable } from "@/components/jira/my-jira/use-jira-availability";
+import { useLinearAvailable } from "@/components/linear/use-linear-availability";
 import { KanbanDisplayDropdown } from "../kanban-display-dropdown";
 import { ReleaseNotesButton } from "../release-notes/release-notes-button";
 import { ReleaseNotesDialog } from "../release-notes/release-notes-dialog";
@@ -88,6 +90,23 @@ function JiraTopbarButton({ workspaceId }: { workspaceId: string | undefined })
   );
 }
 
+function LinearTopbarButton({ workspaceId }: { workspaceId: string | undefined }) {
+  const available = useLinearAvailable(workspaceId);
+  if (!available) return null;
+  return (
+    <Tooltip>
+      <TooltipTrigger asChild>
+        <Button variant="outline" size="icon" asChild className="cursor-pointer">
+          <Link href="/linear">
+            <IconHexagon className="h-4 w-4" />
+          </Link>
+        </Button>
+      </TooltipTrigger>
+      <TooltipContent>Linear</TooltipContent>
+    </Tooltip>
+  );
+}
+
 function ViewToggleGroup({
   toggleValue,
   onValueChange,
@@ -169,6 +188,7 @@ function TabletHeader({
         <TooltipProvider>
           <GitHubTopbarButton />
           <JiraTopbarButton workspaceId={workspaceId} />
+          <LinearTopbarButton workspaceId={workspaceId} />
         </TooltipProvider>
       </div>
       {onSearchChange && (
@@ -250,6 +270,7 @@ function DesktopHeader({
           <TooltipProvider>
             <GitHubTopbarButton />
             <JiraTopbarButton workspaceId={workspaceId} />
+            <LinearTopbarButton workspaceId={workspaceId} />
           </TooltipProvider>
           <Button variant="outline" asChild className="cursor-pointer gap-2">
             <Link href="/stats">