refactor: centralize tool exposure planning

[jif-oai]

Why

Tool exposure is a planning concern, but the deferred MCP path and dispatch-only legacy shell path were carrying those decisions in handler constructors and a shell-only tool-family builder. Keeping those decisions in spec_plan makes the core tool plan easier to follow and keeps handlers focused on runtime behavior.

What changed

• add PlannedTools helpers for ordinary runtimes, exposure overrides, dispatch-only runtimes, and hosted specs
• inline shell tool assembly into core/src/tools/spec_plan.rs and remove the shell-only tool_family module
• remove exposure state and special exposure constructors from McpHandler and ShellCommandHandler
• keep hidden runtime behavior centralized in ExposureOverride, including disabling parallel tool calls for hidden handlers

Testing

• Not run (refactor only)