chore(repo): prepare public contribution settings

[Oneiros667]

Summary

Prepare CodeMesh for a public contribution workflow by adding issue and pull
request templates, a project Code of Conduct, and documented squash-merge title
validation.

Scope

• Included: public contribution templates, conduct expectations, contribution
  guidance, and PR-title Conventional Commit validation.
• Intentionally excluded: repository visibility, public-only security scanning,
  and branch rules that are unavailable while this GitHub Free repository is
  private.

Validation

• Passed: npx --yes markdownlint-cli2@0.23.1 "**/*.md"
• Passed: python tools/check_markdown_links.py
• Passed: python tools/check_publication_safety.py for 194 Git-visible files
• Passed: GitHub YAML parsing
• Passed: python tools/check_conventional_commit.py --range origin/main..HEAD
• Passed: git diff --check origin/main..HEAD
• Pending: hosted pull-request verify workflow

Safety and evidence

• No credentials, private source, customer or employer material, generated
  indexes, databases, model output, or raw evaluation traces are included.
• Documentation and status claims distinguish implementation, automated
  verification, inspected evidence, acceptance, release, and deployment.
• Online providers, live services, and cost-bearing evaluation were not
  used.