fix(artifact): prefer Agentic hook env vars

[tnez]

Summary

• Prefer AGENTIC_ARTIFACT_* env vars from artifact command hooks while relying on the central hook bridge for legacy SPORES_* compatibility.
• Update artifact hook tests to assert both preferred and legacy names are available.
• Refresh the project dogfood artifact.written hook to use Agentic naming first.
• Persist the dogfood output from the v0.5.0 follow-up run: the issue Design: v0.2 composition object (informed by task_next LIFO observation) #16 design artifact, generated artifact memory reference, and local task mirror update.

Dogfood Trail

This came from post-v0.5.0 dogfooding. The run produced a finalized design artifact for #16 and posted the design back to the issue:

#16 (comment)

It also exposed that artifact hook commands were still passing legacy SPORES_ARTIFACT_* names despite the preferred Agentic hook contract.

Follow-up roadmap:

• Roadmap: external artifact storage and cross-environment sync #99 tracks moving internal artifact storage/sync off source-control-backed operational state.
• Roadmap: second-brain example from private dogfood #100 tracks private second-brain dogfooding before extracting a sanitized public example.

Verification

• bun test
• bun run typecheck