Add manage_workspace MCP tool for runtime workspace switching

[jacksandom]

Summary

• Adds manage_workspace MCP tool with status, list, switch, and login actions for runtime workspace switching without restarting the MCP server
• Extends auth.py with session-scoped workspace state (set_active_workspace / get_active_workspace) that overrides get_workspace_client() for all subsequent tool calls
• Rewrites databricks-config skill to route workspace queries through the MCP tool via ToolSearch, solving discoverability with defer_loading: true

Resolves #123

Details

• Switch validates credentials before committing, rolls back on failure
• Detects expired tokens and returns structured response with login instructions
• Login delegates to databricks auth login CLI for OAuth browser flow with stdin=DEVNULL and 120s timeout
• Username cache invalidated on workspace switch to prevent stale identity
• Session-scoped: resets on MCP server restart

Test plan

• auth unit tests (databricks-tools-core/tests/unit/test_auth.py)
• workspace tool tests (databricks-mcp-server/tests/test_workspace.py)
• E2E: "what workspace am I on?" uses MCP tool, not Bash
• E2E: "list my workspaces" returns formatted profile table
• E2E: "switch to [profile]" switches and confirms
• E2E: subsequent tool calls (e.g. list_warehouses) target the new workspace

[calreynolds]

beautiful 👍