feat: add CI validation, index auto-builder, and contributor guides

[ningzimu]

Summary

• Add GitHub Actions workflows for automated PR validation and index auto-update on merge
• Add scripts/build_indexes.py to programmatically generate all index and badge JSON files
• Add pyproject.toml with uv project config and check-jsonschema dependency
• Add AGENTS.md as an AI-agent-oriented contributor guide
• Add dynamic shields.io badge files (firstdata/badges/) for live source count and progress display
• Remove duplicate bis-statistics.json (consolidated into international/economics/bis.json)
• Update .gitignore to exclude .claude/ and CLAUDE.md
• Update README badges to use dynamic endpoint URLs
• Update firstdata/sources/README.md to reference statistics.json instead of hardcoded numbers

CI Workflows

validate-sources.yml (triggered on PR):

1. Block any modification to datasource-schema.json
2. Validate all source JSON files against the schema
3. Check for duplicate id fields across all source files

update-indexes.yml (triggered on merge to main):

1. Re-validate all source files (schema + duplicate IDs)
2. Rebuild all 5 index files and badge JSONs
3. Auto-commit the updated files as firstdata[bot]

Test plan

• uv run check-jsonschema passes on all 131 source files
• scripts/build_indexes.py runs successfully and generates correct output
• Duplicate ID check correctly detected and removed bis-statistics.json
• CI auto-ran update-indexes after merge and committed updated indexes