[codex] open-source baseline

[suchipizza]

What changed

This PR turns the repository into a credible open-source baseline for FEA Copilot.

It adds the public repo surface and contributor scaffolding:

• refreshed README.md
• CODE_OF_CONDUCT.md, SECURITY.md, CONTRIBUTING.md, CHANGELOG.md, ROADMAP.md, CITATION.cff, and AGENTS.md
• issue templates and a PR template
• committed docs, examples, validation assets, and demo/media assets
• markdown link checking and example/benchmark smoke coverage in CI

It also includes the minimal engine and template changes needed to make those repo claims true:

• explicit bracket and plate-with-hole geometry parsing and validation
• analytical screening behavior for bracket and plate-with-hole mock workflows
• explicit Docker rejection for unsupported geometries instead of vague fallthrough
• committed simply supported beam, Roark-style plate, and mesh-convergence baseline assets
• template and golden-script updates for the current beam/plate load handling

Why it changed

The repo was already close to open-source ready, but the first public baseline needed a cleaner surface and reproducible evidence.

This branch publishes the launch-ready layer now instead of waiting for follow-up roadmap work, while intentionally leaving unrelated local core edits out of the PR.

Impact

For users:

• clearer project scope and limitations
• reproducible examples and validation entry points
• explicit behavior for unsupported bracket and plate-with-hole Docker workflows

For contributors:

• clearer contribution paths
• benchmark and adapter scaffolds
• CI that exercises docs links, smoke examples, and the analytical benchmark path

Validation

Validated from this branch state:

• python3 -m py_compile app.py fea_engine/*.py templates/*.py tests/*.py tools/*.py validation/mesh_convergence/*.py validation/roark_formulas/*.py
• pytest -q
• python3 tools/check_markdown_links.py
• ./examples/smoke_test.sh
• ./validation/roark_formulas/run.sh

[suchipizza]

Superseded by #3 after renaming the publish branch to hellostress/open-source-baseline for cleaner long-lived naming. Closing this PR to keep review on the renamed branch.