Add ToolCall struct and extend ToolExecutor trait

[bug-ops]

Parent: #239 (Phase A)

Summary

Add structured ToolCall type and extend ToolExecutor with tool_definitions() and execute_tool_call() methods.

Changes

• crates/zeph-tools/src/executor.rs: add ToolCall { tool_id, params } struct
• Extend ToolExecutor trait with fn tool_definitions(&self) -> &[ToolDef] and fn execute_tool_call(&self, call: &ToolCall) -> Result<Option<ToolOutput>, ToolError>
• Keep existing execute(&self, response: &str) for backward compat (dual-mode)

Acceptance criteria

• ToolCall struct with serde support
• ToolExecutor trait backward-compatible extension
• Existing bash extraction path unchanged