release: prepare v0.4.0

[ElvinMorales]

Closes #68

Summary

• bump package metadata and __version__ to 0.4.0
• add the v0.4.0 changelog entry for the LLM-layer/runtime-prototype workstream
• rebuild the release checklist for v0.4.0
• remove stale current-v0.3 wording from README and supporting docs
• add low-risk CI smoke checks for runtime-wrapper proposals and review-summary JSON syntax
• note that external PR perf(scanner): optimize file inventory tracking with O(1) sets and ze… #62 is outside the v0.4 release scope
• keep runtime behavior, deterministic schemas, generated outputs, publishing automation, and package release behavior unchanged

Validation

• python -m pip install -e ".[dev]"
• agent-librarian --version
• agent-librarian --help
• agent-librarian catalog --help
• agent-librarian catalog examples/sample-collection --out examples/generated-catalog
• deterministic regeneration with SOURCE_DATE_EPOCH=1781481600
• agent-librarian validate examples/generated-catalog
• agent-librarian report examples/generated-catalog
• python -m agent_librarian.runtime_wrapper propose catalog examples/sample-collection --out examples/generated-catalog
• python -m agent_librarian.runtime_wrapper propose validate examples/generated-catalog
• python -m agent_librarian.runtime_wrapper propose report examples/generated-catalog
• wrong-command wrapper approval check, expected failure with exit code 2
• python -m json.tool agent/schemas/review-summary.schema.json
• python -m json.tool agent/schemas/review-summary.example.json
• YAML parse checks for agent/tools/tools.yaml and CI workflow
• pytest
• git diff --check
• git diff --exit-code -- examples/generated-catalog

Notes

• pytest: 52 passed
• Generated outputs are unchanged
• Runtime behavior is unchanged
• Deterministic schemas are unchanged
• No push, tag, release, PyPI publish, or gh command was run by Codex
• External PR perf(scanner): optimize file inventory tracking with O(1) sets and ze… #62 remains outside v0.4 and should be reviewed separately after release