feat(cmd): execute vocab validate in standards repo working dir

[mdheller]

Summary

This PR advances the vocab runtime surface by adding real local execution for prophet vocab validate.

Included

• internal/exec/runner.go supports execution in a specified working directory
• internal/vocab/vocab.go resolves a standards repo path and runs policy/tools/validate_all.py there
• internal/cmd/vocab.go now emits the returned result payload even on validator failure
• tests for repo-path resolution and default graph-path behavior

Behavior

prophet vocab validate now:

1. resolves the standards repo path from PROPHET_VOCAB_REPO, or defaults to ../socioprophet-standards-knowledge
2. defaults graph paths to . when none are provided
3. executes python3 policy/tools/validate_all.py --data ... in the standards repo working directory
4. returns structured result data and a non-zero error when validation fails

Why

The standards chain in socioprophet-standards-knowledge is now staged through PRs #28-#31. The runtime CLI needs to be able to invoke that validator contract for real, not merely advertise it.

Note on review chain

This branch was originally intended to stack on top of #12. The live feat/vocab-runtime-plumbing branch is not currently visible in the repository branch list, so this PR is opened against feat/vocab-surface as the nearest live review base. Functionally, it carries the runtime-plumbing step forward and can supersede #12 if needed.

Scope

Still intentionally narrow:

• no CI/build hook changes yet
• no K8s shapecheck yet
• no ABD validation wrapper yet