On-demand context retrieval for DeepSeek Harness

Key features

• sessionQuery and subprocess are optional services read via ctx.get(...).
• A missing sessionQuery backend degrades each tool to an isError result (session query backend unavailable) instead of leaving the plugin waiting forever
• the same applies to subprocess for context_grep with an explicit path.
• tools is a hard dependency (inject).
• The packaged ripgrep binary comes from @vscode/ripgrep — the same mechanism dsh-tool-fs-search uses — so no system rg install is required and no shell layer exists between the argv vector and ripgrep.