perf(store): keep the repo list's identity through workspace hydration

buildRuntimeSessionPlaceholders opened with `repos.slice()`, so every workspace
session hydration handed the store a brand-new `repos` array — including the
common case where the session referenced no unknown runtime workspace and the
contents were identical. `repos` is selected whole at 46 sites, so each
hydration rerendered all of them for no data change.

The appends below already build a new array rather than mutating, and the
sibling `nextWorktreesByRepo` in the same function was already copy-on-write;
this just gives `repos` the same treatment. No consumer of the returned array
mutates it in place.

src/renderer/src/store/terminals/workspace-terminal-placeholders.test.ts

@@ -0,0 +1,83 @@
+import { describe, expect, it } from 'vitest'
+import type { Repo } from '../../../../shared/repo-types'
+import type { Worktree } from '../../../../shared/worktree/types'
+import { DEFAULT_REPO_BADGE_COLOR } from '../../../../shared/constants'
+import { buildRuntimeSessionPlaceholders } from './workspace-terminal-placeholders'
+
+const repo: Repo = {
+  id: 'repo-1',
+  path: '/repos/one',
+  displayName: 'one',
+  badgeColor: DEFAULT_REPO_BADGE_COLOR,
+  addedAt: 0,
+  connectionId: null,
+  executionHostId: 'local'
+}
+
+const worktree: Worktree = {
+  id: 'repo-1::/repos/one',
+  repoId: 'repo-1',
+  hostId: 'local',
+  displayName: 'main',
+  comment: '',
+  linkedIssue: null,
+  linkedPR: null,
+  linkedLinearIssue: null,
+  linkedGitLabMR: null,
+  linkedGitLabIssue: null,
+  isArchived: false,
+  isUnread: false,
+  isPinned: false,
+  sortOrder: 0,
+  lastActivityAt: 0,
+  path: '/repos/one',
+  head: '',
+  branch: '',
+  isBare: false,
+  isMainWorktree: true
+}
+
+describe('buildRuntimeSessionPlaceholders', () => {
+  it('returns the original repos array when no placeholder repo is needed', () => {
+    const repos = [repo]
+    const worktreesByRepo = { 'repo-1': [worktree] }
+
+    const result = buildRuntimeSessionPlaceholders({
+      repos,
+      runtimeHostIdByWorkspaceSessionKey: {},
+      worktreesByRepo
+    })
+
+    // Hydration writes these straight to the store; a fresh array would rerender
+    // every component selecting the whole repo list for no data change.
+    expect(result.repos).toBe(repos)
+    expect(result.worktreesByRepo).toBe(worktreesByRepo)
+  })
+
+  it('keeps the original repos array when the session only references known repos', () => {
+    const repos = [repo]
+
+    const result = buildRuntimeSessionPlaceholders({
+      repos,
+      runtimeHostIdByWorkspaceSessionKey: { 'repo-1::/repos/one': 'runtime:host-1' },
+      worktreesByRepo: { 'repo-1': [worktree] }
+    })
+
+    expect(result.repos).toBe(repos)
+  })
+
+  it('still appends a placeholder repo for an unknown runtime workspace', () => {
+    const repos = [repo]
+
+    const result = buildRuntimeSessionPlaceholders({
+      repos,
+      runtimeHostIdByWorkspaceSessionKey: { 'repo-2::/repos/two': 'runtime:host-1' },
+      worktreesByRepo: { 'repo-1': [worktree] }
+    })
+
+    expect(result.repos).not.toBe(repos)
+    expect(result.repos.map((entry) => entry.id)).toEqual(['repo-1', 'repo-2'])
+    // The caller's array must not be mutated in place.
+    expect(repos).toHaveLength(1)
+  })
+})

src/renderer/src/store/terminals/workspace-terminal-placeholders.ts

@@ -20,10 +20,15 @@ export function buildRuntimeSessionPlaceholders({
   runtimeHostIdByWorkspaceSessionKey: Record<string, ExecutionHostId>
   worktreesByRepo: Record<string, Worktree[]>
 }): {
-  repos: Repo[]
+  repos: readonly Repo[]
   worktreesByRepo: Record<string, Worktree[]>
 } {
-  let nextRepos = repos.slice()
+  // Why not `repos.slice()`: hydration writes this array straight back to the store, and
+  // sessions that add no placeholder repo are the common case. Copying unconditionally
+  // gave `repos` a new identity on every workspace hydration, rerendering every component
+  // that selects the whole array. Appends below already build a new array, as
+  // nextWorktreesByRepo does for its own copy-on-write.
+  let nextRepos: readonly Repo[] = repos
   let nextWorktreesByRepo = worktreesByRepo
   for (const workspaceSessionKey of Object.keys(runtimeHostIdByWorkspaceSessionKey)) {
     const hostId = runtimeHostIdByWorkspaceSessionKey[workspaceSessionKey]