#20843: docs: add troubleshooting for A2UI bundling and missing node_modules

Summary • Problem: A2UI bundling can fail with node_modules missing / exit code 254 when dependencies aren’t installed from the workspace root. • Why it matters: New contributors hit a hard stop during setup; the error looks like a build problem but is actually a missing install step / wrong working directory. • What changed: Added a Troubleshooting section documenting the error signatures and the exact fix steps (pnpm install from workspace root + clean reinstall fallback). • What did NOT change (scope boundary): No runtime behavior, code, build output, or dependencies changed—documentation only. Change Type (select all) • Docs Scope (select all touched areas) • UI / DX Linked Issue/PR • Related # (optional) N/A User-visible / Behavior Changes Documentation only: added troubleshooting instructions for A2UI bundling/setup. No behavior changes. Security Impact (required) • New permissions/capabilities? No • Secrets/tokens handling changed? No • New/changed network calls? No • Command/tool execution surface changed? No • Data access scope changed? No • If any Yes, explain risk + mitigation: N/A Repro + Verification Environment • OS: macOS • Runtime/container: Node.js + pnpm workspace • Model/provider: N/A • Integration/channel (if any): N/A • Relevant config (redacted): N/A Steps 1. Fresh clone the repo. 2. Run pnpm canvas:a2ui:bundle without installing dependencies from the workspace root. 3. Observe error: node_modules missing / bundling failure (exit code 254). 4. Run the documented fix: • corepack enable • corepack prepare pnpm@latest --activate • pnpm install (from workspace root) • pnpm canvas:a2ui:bundle Expected • Bundling succeeds after installing deps from workspace root; clean install fallback works if needed. Actual • Bundling failed before; succeeded after following the documented steps. Evidence • Trace/log snippets Human Verification (required) • Verified scenarios: Reproduced the failure on a fresh clone and confirmed the documented steps resolve it. • Edge cases checked: Clean reinstall path (rm -rf node_modules **/node_modules + pnpm store prune) resolves persistent failures. • What you did not verify: No code-path/runtime verification (docs-only change). Compatibility / Migration • Backward compatible? Yes • Config/env changes? No • Migration needed? No • If yes, exact upgrade steps: N/A Failure Recovery (if this breaks) • How to disable/revert this change quickly: Revert the documentation commit. • Files/config to restore: N/A • Known bad symptoms reviewers should watch for: N/A Risks and Mitigations None. <!-- greptile_comment --> <h3>Greptile Summary</h3> Added troubleshooting documentation for A2UI bundling failures. The new section documents three common error signatures (`node_modules missing`, bundling failure message, and exit code 254) and provides step-by-step fixes including corepack setup, dependency installation from workspace root, and a clean install fallback. The troubleshooting section is appropriately placed after the channel configuration sections and before the main docs links section. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - Documentation-only change that adds helpful troubleshooting guidance. The documented commands are accurate (verified `pnpm canvas:a2ui:bundle` exists in package.json and `pnpm-workspace.yaml` exists at repo root). The error messages and fixes align with common pnpm workspace setup issues. No code, dependencies, or runtime behavior changed. - No files require special attention _{Last reviewed commit: 33bb47d} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->