Update CompositeExecutor for tool routing

[bug-ops]

Parent: #239 (Phase A)

Summary

Extend CompositeExecutor to aggregate tool definitions from sub-executors and route ToolCall by tool_id.

Changes

• crates/zeph-tools/src/composite.rs: aggregate tool_definitions() from all sub-executors
• Route execute_tool_call() to matching executor by tool_id
• Error on unknown tool_id

Acceptance criteria

• CompositeExecutor implements extended ToolExecutor trait
• Routing dispatches to correct sub-executor
• Unknown tool_id returns clear error
• Unit tests for routing