Prepare 0.3.0 release metadata

[fsistemas]

Summary

Prepares the public 0.3.0 release metadata and release-facing docs.

• Bumps package metadata and lockfile from 0.2.1 to 0.3.0.
• Adds 0.3.0 changelog notes for connection-scoped named queries, scoped-over-global precedence, backwards-compatible global queries, and scoped discovery changes.
• Updates release/version examples in public docs and maintainer release docs.
• Clarifies that connection_queries is optional and existing configs without it continue to work unchanged.
• Updates Python API example docs to mention scoped discovery and legacy-config behavior.
• Syncs the canonical sql2json agent skill.

Validation

Backwards compatibility with a config that omits connection_queries:

• .venv/bin/python -m sql2json --config /tmp/sql2json-legacy-config.json --list-connections -> ["default"]
• .venv/bin/python -m sql2json --config /tmp/sql2json-legacy-config.json --list-queries -> {"global": ["answer", "plus"], "connections": {}}
• .venv/bin/python -m sql2json --config /tmp/sql2json-legacy-config.json --list-queries legacy -> ["answer", "plus"]
• .venv/bin/python -m sql2json --config /tmp/sql2json-legacy-config.json --name default --query answer -> [{"answer": 42}]
• Python API legacy config check: list_queries, list_queries(scoped=True), list_queries(connection="default"), run_query_by_name, and run_query2json all passed.

Docs/examples verification:

• for f in examples/python_api/*.py; do .venv/bin/python "$f"; done — all examples ran successfully
• bash scripts/sync-sql2json-skill.sh — completed
• python - <<'PY' ... pyproject version ... PY — printed 0.3.0

Quality gates:

• .venv/bin/python -m pytest -q — 138 passed, 12 deselected
• .venv/bin/black --check sql2json tests examples — passed
• .venv/bin/flake8 sql2json tests examples — passed
• .venv/bin/python -m mypy sql2json — passed