Add workspace history with :workspace-history picker

[WantenMN]

Records recently opened workspace roots so they can be listed and jumped back to, similar to VS Code's Open Recent.

What this adds:

• New helix-loader::workspace_history module storing an array of {path, last_seen} at data_dir()/workspace_history.json (atomic write, best-effort: IO failures only warn, never break :cd or startup). Prunes missing directories, caps entries.
• Recording hooks: Editor::set_cwd (covers :cd/:pushd/:popd) and application startup.
• [editor.workspace-history] config (enable, max-entries).
• Picker + commands: normal-mode workspace_history_picker, typed :workspace-history and :workspace-history-clear. Bare queries match a combined name + path primary column (bare text only filters the primary column, see PickerQuery); opened column is display-only.
• Docs regenerated via cargo xtask docgen, plus an editor.md section.

Testing:

• 6 new unit tests in workspace_history (upsert ordering, truncation, pruning, broken-file tolerance, disabled noop).
• cargo check -p helix-term, cargo fmt clean.
• Manual pty smoke tests: startup record, :cd record, picker lists entries and filters by both name and path fragments, clear removes the file.