chore: add GitHub community kit, templates and CHANGELOG

[abhinav-phi]

Summary

Adds the complete GitHub community kit — the standard open-source scaffolding (per the widely-used project-template layout) tailored to The Sentinel NIDS — plus a CHANGELOG covering both the current Colab Edition and the pre-Colab local edition history.

What's Included (12 files)

Governance & community

File	Purpose
.github/SECURITY.md	Private vulnerability-disclosure policy (GitHub Security Advisories), scoped to the embedded API, artifact integrity & notebook code paths; documents in-scope vs out-of-scope and known accepted limitations
.github/CONTRIBUTING.md	The project's golden rule — edit _build/*.py sources, never the .ipynb — plus the full workflow (pack.py → smoke_test.py), where-changes-go table, code standards (determinism, artifact contract, no silent coercion, graceful GPU fallback)
.github/CODE_OF_CONDUCT.md	Contributor Covenant v2.1 with enforcement guidelines
.github/SUPPORT.md	Docs index, quick troubleshooting table (dataset/artifact/GPU issues), and escalation paths
.github/CODEOWNERS	Reviewer routing (maintainer by default; _build/ and docs/ highlighted)
.github/FUNDING.yml	Sponsor-button scaffold (commented placeholders — enable a platform to activate)

Templates

File	Purpose
.github/pull_request_template.md	Type/area checklists, CI-first checklist (pack.py + smoke_test.py), artifact-contract and determinism reminders, conventional-commit title rule
.github/ISSUE_TEMPLATE/1-bug-report.md	Notebook/cell/runtime-scoped bug report with traceback block
.github/ISSUE_TEMPLATE/4-feature-request.md	Motivation → proposal → alternatives, with a contribution opt-in
.github/ISSUE_TEMPLATE/config.yml	Blank-issues off; routes Questions → Discussions and Security → private advisories

Automation & history

File	Purpose
CHANGELOG.md	Keep-a-Changelog format: [1.0.0] Colab Edition (notebook suite, build system, fixes for ISSUE-05/06, local-code removal) + the [0.x] local-edition history
.github/workflows/ci.yml (second commit — see note)	GitHub Actions: rebuild + nbformat-validate all four notebooks and run the end-to-end smoke test on every push/PR

Why

The repo previously had no contribution surface beyond the README. This kit makes the project look and behave like a properly-governed open-source project — safe disclosure, guided contributions, structured issues, and automated validation — which matters for a security-oriented portfolio project.

Note on the CI workflow

The stored OAuth credential lacks the workflow scope, so .github/workflows/ci.yml is committed locally on the same branch but could not be pushed by automation. Merging this PR first, then pushing the branch's second commit (git push origin chore/github-community-kit after git pull), or applying the identical file manually, enables CI. The file is ready-to-use as-is (working-directory–scoped steps, pip cache, 20-minute timeout).

Testing

• All Markdown renders cleanly (tables, admonitions, relative links verified against the tree)
• config.yml link targets exist (/discussions, /security/advisories/new, /docs)
• CODEOWNERS uses plain * + directory rules valid for a solo-maintained repo
• CHANGELOG link references match Keep-a-Changelog conventions