## Why
`ToolExecutor` is the runtime contract that keeps a callable tool and
its model-visible spec together. Leaving `spec()` optional lets a
registered runtime silently omit that half of the contract, and it also
overloads a missing spec as an exposure decision for tools that should
stay dispatchable without being shown to the model.
## What
- Make `ToolExecutor::spec()` required and update core, extension, and
test tool executors to return a concrete `ToolSpec`.
- Add `ToolExposure::Hidden` for dispatch-only tools. The legacy
`shell_command` runtime in unified-exec sessions now uses that explicit
exposure instead of hiding itself by omitting a spec.
- Build MCP tool specs when `McpHandler` is constructed so invalid MCP
specs are skipped before the handler is registered.
- Keep tool planning aligned with the new contract for direct, deferred,
hidden, code-mode, dynamic, and namespaced tool paths.
## Testing
- Added tool-plan coverage that invalid MCP tool specs are not
registered.
- Updated shell-family coverage for the hidden legacy `shell_command`
runtime and the affected tool executor test fixtures.
516f134641