fix: read HookResult modify action from tool:post events

[bkrabach]

Summary

Fixes the orchestrator to read modified tool results from tool:post hook events. Previously, when a hook returned HookResult(action="modify", data={"result": ...}), the modified data was ignored and the original result.get_serialized_output() was used instead.

Problem

The hook protocol's modify action works correctly in the hook layer — hooks.emit() processes the modification and returns the updated data. But the orchestrator never checked if the data was modified, always using the original tool result.

This broke any hook that modifies tool output (truncation, sanitization, transformation). The hooks-tool-truncation module was completely ineffective because its modifications were silently discarded.

Fix

After hooks.emit("tool:post", ...), detect if the returned data differs from the original (identity comparison) and use the modified result when a hook has changed it. Falls back to get_serialized_output() when no modification occurred (backward compatible).

Testing

• 3 new tests: modify replaces result, no-modify uses original, string result handling
• 26/26 total tests pass
• Shadow-validated across 7 repos (753/753 total)

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