Improve session permissions, deletion, and workspace file management

[lukebuehler]

Adds optional session soft deletion and deployment-admin purge, tightens contributor session configuration permissions, and improves workspace file management.

• Keep direct API and CLI deletion permanent by default; use soft deletion in Platform and retention, with audited admin purge and deleted-session management.
• Separate configuration, close, delete, and retention permissions across runtime and Platform, and refresh development API key grants.
• Add workspace drag-and-drop moves and Markdown preview controls, with corresponding API consumers, tests, schema migration, and documentation.

Validation: workspace Clippy with warnings denied, Rust formatting, full npm run check, and release metadata verification passed. Scoped Rust tests and documentation checks were started and were still running when this PR was opened. Live tests were not run.