Saved tools and code mode in the agent editor

Two parity gaps between code-first and dashboard workflows are closed.

• Saved custom tools are visible. Code and HTTP tools created through the API, MCP, or SDK now appear as rows in the agent editor's tool-selection modal, so they can be equipped from the dashboard. Tools that are equipped but inactive stay visible with an "Inactive" label so stale references can be cleaned up.
• Code mode in the sidebar. A new Code mode section edits the agent's code-mode configuration: the toggle, the tool pool, and the sandbox timeout. In-editor test runs carry the same configuration the saved agent runs with. Wildcard pool entries like prefix:* survive dashboard edits, so a dashboard save never narrows a pool authored over the API.

One API note: the agent update endpoint now accepts an explicit null for subagentConfig and codeModeConfig, which is the way to remove a saved block (omitted keys are preserved by the field merge).