Deprecate string-based ToolExecutor::execute in favor of structured tool calls

[bug-ops]

Part of #391

Problem

ToolExecutor::execute() takes response: &str and parses tool invocations from text. This is a legacy path from before chat_with_tools(). Both string-based and structured dispatch paths must be maintained.

File: crates/zeph-tools/src/executor.rs line 92

Proposed Solution

After all providers support chat_with_tools(), deprecate the string-based execute() method.

Priority: P3 | Effort: Medium