feat(mcp): expose ScriptedTool as MCP tool

Refactor MCP server into McpServer struct with register_scripted_tool().
ScriptedTools appear in tools/list and route tools/call correctly.
Keep existing bash tool for backward compat.

Gate behind scripted_tool feature flag on bashkit-cli.
9 tests including full JSON-RPC roundtrip. Update spec 014.

Closes #520

Author: chaliy

crates/bashkit-cli/Cargo.toml

@@ -22,6 +22,7 @@ doc = false  # Disable to avoid collision with bashkit library docs
 default = ["python"]
 python = ["bashkit/python"]
 realfs = ["bashkit/realfs"]
+scripted_tool = ["bashkit/scripted_tool"]
 
 [dependencies]
 bashkit = { path = "../bashkit", version = "0.1.9", features = ["http_client", "git"] }