emitLlmArtifacts previously emitted ArtifactCreated/Validating/Validated for every
llm-emitted slot unconditionally, fabricating "validated" artifacts when the model
produced no content. Gate now mirrors the F-015 file_written check: slots with null
or blank cached content are skipped with a warn log. Regression test added in
NeedsArtifactInjectionTest verifies no artifact events appear for a blank-response stage.
Stages declaring needs=[...] now receive each needed artifact's validated
JSON (from artifactContentCache) as an L1/USER neededArtifact context entry,
so a one-shot subagent sees its inputs.
Also emit llm-emitted artifact lifecycle events (Created/Validating/Validated)
on successful validation via emitLlmArtifacts, closing a latent gap where
verifyProduces would fail for stages producing only an llm-emitted artifact.
Events are emitted only after validation passes, preserving invariant #7.
kami