feat(worksets): add list, detail, and create flow

[RandyZ]

Summary

• Add Worksets list, detail, and create flows with trusted Project/Store binding and one-time opener overrides.
• Keep Workset CLI calls selector-free, add shell-free Windows launcher resolution, and preserve existing state on validation failures.
• Archive the OpenSpec change, sync the four affected specs, and include real Extension Host evidence for dark, light, and high-contrast themes.
• Prepare the feature release as v0.2.1: update CHANGELOG, English/Chinese marketplace README content, package description, and Store/Workset discovery keywords.
• This branch also contains the earlier Worksets design/planning commits, the structured spec viewer plan, and the dashboard article draft.

Test plan

• pnpm test — 63 files / 1005 tests passed
• pnpm run build
• pnpm run package — generated and verified openspec-workflow-0.2.1.vsix
• Packaged manifest reports version 0.2.1 and packaged README contains the Stores and Worksets guide
• Changed OpenSpec specs validate successfully
• Real Extension Development Host checked at 430px in dark, light, and high-contrast themes

Notes

openspec validate --all still reports three unrelated pre-existing repository issues: add-i18n-support, extension-state-storage, and marketplace-publish.