build: upgrade Turborepo to 2.11

[nealrichardson]

Phase 0.4 of the dependency upgrade. It's independent of #1149 (Node 24) and #1150 (TypeScript 5.9). The only overlap is that this PR and #1149 both edit the root package.json, which is a trivial merge.

Changes

• turbo ^1.8.5 → ^2.11.4. In turbo.json, pipeline becomes tasks, the $schema is https://turborepo.dev/schema.json, and dev gets persistent: true (every dev script is a watcher).
• Root package.json: "packageManager": "yarn@1.22.22". Turbo 2 refuses to run without it, and the codemod doesn't add it. With corepack enable, this also pins Yarn to 1.22.22.
• Removed eslint-config-turbo from eslint-config-custom-server. Nothing extends it, and its turbo >2 peer was unmet under turbo 1. The lockfile also drops eslint-plugin-turbo and dotenv.
• VS Code tasks (apps/vscode 'Run Extension' pre-launch task calls a nonexistent watch script #1141). Both apps/vscode/.vscode/tasks.json (the Run Extension pre-launch task) and the root .vscode/tasks.json ran a watch script that doesn't exist. Both now run yarn dev-vscode from the repo root. $tsc-watch never matches esbuild output, so it's replaced with a background matcher: it begins on turbo's Packages in scope line and ends on quarto:dev: [watch] build finished. I checked both patterns against real dev-vscode output.
• apps/vscode/CONTRIBUTING.md: the debug steps named the root launch configs (Run VS Code Extension, Attach to VS Code LSP Server) but said to open apps/vscode. They now say to open the repo root, and mention Run Extension from apps/vscode as an alternative.
• CLAUDE.md: "pipeline" wording → "task".

Not included: the optional panmirror#build env task (PANMIRROR_OUTDIR; RStudio runs panmirror's build directly, not through turbo) and the cross-package outputs fix (#1144). I kept strict env mode: the only build-time env read inside a turbo task is PANMIRROR_OUTDIR.

Verification (local, Node 24.21.0, Yarn 1.22.22)

• --filter quarto... --dry=json selects apps/vscode plus its dependencies. It doesn't select the root, even though the root is also named quarto. --filter='./packages/*' selects the same 5 test suites as before.
• yarn build: 14/14 tasks. A second run gets 14 cached, FULL TURBO. The pre-existing no output files found for task quarto-lsp#build warning is Turbo: lsp/markdownit/editor builds write outside declared outputs, so cache hits don't restore them #1144.
• yarn build-vscode: out/main.js, out/lsp/lsp.js, out/markdownit/ and assets/www/editor/index.js are all present.
• cd apps/vscode && yarn run vscode:prepublish, then vsce package: produces a VSIX.
• yarn dev-vscode: all four watchers (quarto, quarto-lsp, quarto-vscode-editor, quarto-vscode-markdownit) start, and stopping it leaves no stray processes. I didn't exercise F5 through the new pre-launch task interactively, so that's worth a quick try in review.
• yarn lint --continue: same as main. The only failures are the 6 pre-existing no-unused-vars errors in quarto-lsp.
• yarn test-packages: 8/8 tasks.
• yarn test-vscode (VS Code 1.109, Quarto CLI on PATH): main label 150 passing, 0 failing; r-project label 3/3.

Fixes #1141.

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