fix(historian): surface empty completed native task results (#270 diagnostics)

Extracted from PR #337 (Cole Leavitt). A completed native task tool can
return an empty <task_result> when its provider emitted reasoning only or
context-fill truncation dropped the final output. Annotate such outputs
with a diagnostic sentinel from the tool-execute-after hook so the caller
can distinguish an empty result from a genuinely-empty tool.

Co-Authored-By: Cole Leavitt <cole@unwrap.rs>

Co-Authored-By: Alfonso <alfonso@cortexkit.io>