Add GitHub Actions workflow for COBOL compilation checks

[devin-ai-integration]

Add GitHub Actions workflow for COBOL compilation checks

Summary

Adds CI to verify that COBOL examples still compile. The workflow (.github/workflows/compile-check.yml) triggers on push/PR to main and does two things:

1. Syntax check — runs cobc -fsyntax-only on all .cbl files except sql/ (excluded because EXEC SQL blocks require the esqlOC precompiler to parse)
2. Full compilation — compiles 7 non-interactive examples (json_generate, xml_generate, numval_test, comp_test, trim, is_numeric, merge_sort) and verifies they produce executables

Skipped entirely (with documented reasons in the workflow):

• sql/ — requires esqlOC precompiler and libocsql (not available in CI); excluded from both syntax check and compilation
• accept/, mouse/, screen_size/ — require interactive terminal input; skipped from compilation but still syntax-checked

Also adds a CI status badge to the top of README.md.

Updates since last revision

• Excluded sql/ from the syntax-check step as well (not just the compile step). The EXEC SQL blocks cause cobc -fsyntax-only to fail without the esqlOC preprocessor. CI now passes.

Review & Testing Checklist for Human

• Confirm the compiled examples set is correct: display_test, display_timing, read_command_args, report_writer, search, sub_program, unstring, redifines are currently only syntax-checked, not compiled. Verify this is the desired split — some of these may be safe to add to the compile step.
• Verify the CI badge renders on README: After merging, confirm the badge at the top of README.md links to the correct workflow and shows a passing status.
• Consider pinning ubuntu-latest: The workflow uses ubuntu-latest (currently Ubuntu 24.04, GnuCOBOL 3.1.2). If the runner image or packaged GnuCOBOL version changes in the future, compilation behavior could change. Pinning to ubuntu-24.04 may be more stable.

Suggested test plan: Merge this PR, then push a trivial commit to main (or open a test PR) and confirm the workflow runs and passes end-to-end. Verify the badge updates on the README.

Notes

• Requested by: @rdf004
• Link to Devin run

[devin-ai-integration]

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