Introduce tool exposure for deferred registration

[jif-oai]

Why

Deferred tools were tracked with separate side-channel filtering after tool specs had already been assembled. That made the registry responsible for executing tools while the router/spec planner separately decided whether those same tools should be exposed to the model up front.

This PR makes exposure part of the tool handler contract so direct versus deferred availability travels with the executable tool registration.

Next step will be to simplify registration

What Changed

• Adds ToolExposure to codex-tools and exposes it through ToolExecutor, defaulting tools to Direct.
• Teaches dynamic tools and MCP handlers to mark deferred tools as Deferred at construction time.
• Renames the registry object-safe wrapper from AnyToolHandler to RegisteredTool and uses ToolExposure when deciding whether to include a handler's spec in the initial model-visible tool list.
• Refactors tool spec planning to derive direct specs and deferred search entries from registered handlers, removing the router's special-case deferred dynamic tool filtering.

Verification

• Not run.