Add public contribution templates

[jmcte]

Summary

• add structured bug and feature issue forms, disable blank issues, and point security reports at SECURITY.md
• add a PR template with R milestone, test plan, docs, and fixture/privacy checklist prompts
• align CODEOWNERS with the manifest rule for * @jmcte
• add a good-first-issue contribution entry point
• update live repo metadata: description, README homepage, topics, and missing feature label

Verification

• bash scripts/ci/run-fast-checks.sh
• gh repo view OMT-Global/mailplus-intelligence --json description,homepageUrl,isPrivate,repositoryTopics,url
• gh label list --repo OMT-Global/mailplus-intelligence --limit 100 --json name | jq -r '.[].name' | rg '^(release-v0.1|bug|feature|documentation|epic|good first issue|help wanted)$'

Notes:

• Repo visibility is already public.
• I did not add CODE_OF_CONDUCT.md; the repo now has SECURITY.md, CONTRIBUTING.md, structured issue forms, and CODEOWNERS for the v0.1 public surface. A separate conduct policy can be added when the organization standardizes one.
• The actual v0.1.0 tag/release remains gated on R1-R5 landing first.

Closes #87.

[pheidon]

Approved from Pheidon second-pass review. I found no blocker in the contribution templates/CODEOWNERS update; local secret scan passed and referenced template labels exist. Merge should still wait for required GitHub CI to complete.