Adopt marker-tab window model for Workspaces, drop tab-groups support

[trsrm]

Context & Rationale

Chrome/Brave now persist Tab Groups, which caused duplicate markers and broke the original “tab-group-per-workspace” approach. This PR finishes the move to a pure window-based model: every workspace lives in its own browser window, identified by a pinned “marker tab”. The marker tab gives us a reliable window identifier (even after browser restarts or extension reloads) and doubles as a mini status page. As part of this change we cleaned up the docs, welcome page, and manual test plan so they no longer mention Tab Groups, removed the unreliable toolbar badge indicator, added small UX niceties (clipboard helper, marker self-healing), and cut a release prep pass for 0.8.

Highlights

• Window-only workspaces: Opening a workspace now always prepends a pinned marker tab; syncing ignores marker tabs; background scripts rebind windows from markers at startup and recreate markers if they’re closed manually.
• Marker page UX: The marker HTML page now shows workspace info, a color-coded favicon, and a “Copy workspace tabs” button that writes every tab’s title/URL to the clipboard.
• Docs & onboarding: README, Welcome page, and manual Test Plan now describe the dedicated-window + marker-tab experience (no more “tab group” language), including marker behavior in test expectations.

[trsrm]

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