chore(repo): fix license detection and add github workflows

LICENSE now starts with the EUPL-1.2 standard header so github-linguist
detects the licence correctly in the repo header. The dual-copyright
block (upstream ChatTwo authors plus Hellion Online Media) moves into a
new COPYRIGHT file referenced from the README. NOTICE.md and
UPSTREAM_SYNC.md stay as-is.

New files under .github:

- workflows/build.yml: validates every push to main and every PR
  against the current Dalamud staging branch on a Windows runner
- workflows/release.yml: builds Release on every v* tag, locates the
  DalamudPackager latest.zip and attaches it to the matching GitHub
  Release via softprops/action-gh-release
- dependabot.yml: weekly NuGet sweeps and monthly GitHub Actions
  sweeps with conventional-commit prefixes, grouped patch and minor
  PRs to cut review noise
- ISSUE_TEMPLATE/bug_report.yml + feature_request.yml + config.yml:
  structured intake that pushes security reports through the private
  advisory flow and routes upstream-only issues to ChatTwo
- SECURITY.md: documents the vulnerability reporting channels, scope,
  and target disclosure window

The release workflow replaces the previous manual upload step. Tag a
release and the ZIP shows up on the release page automatically.

COPYRIGHT

@@ -0,0 +1,27 @@
+HellionChat — a privacy-focused fork of ChatTwo for FINAL FANTASY XIV
+
+Copyright (c) 2024-2025 Infiziert90 (Infi) and Anna Clemens (ascclemens)
+    Original ChatTwo authors and copyright holders of the upstream
+    plugin this fork is built on. Their work covers the message store,
+    the channel filtering, the sidebar tab system, the FFXIV chat
+    hooks, the localisation infrastructure and most of the
+    architecture HellionChat still relies on.
+
+Copyright (c) 2025-2026 Florian Wathling / Hellion Online Media
+    HellionChat-specific modifications, including the privacy filter,
+    per-channel retention sweep, export pipeline, Auto-Tell-Tabs,
+    Hellion theme and font integration, German localisation and the
+    EUPL-1.2 fork maintenance.
+
+Licensed under the European Union Public Licence (EUPL), Version 1.2
+only. The full Licence text lives in the LICENSE file at the root of
+this repository. The official Licence website is at:
+
+    https://eupl.eu/1.2/en/
+
+This Work is provided "AS IS" without warranties of any kind. See
+Article 7 (Disclaimer of Warranty) and Article 8 (Disclaimer of
+Liability) of the Licence for the legally binding wording.
+
+Acknowledgements directed at the upstream ChatTwo authors live in
+NOTICE.md. The manual upstream-sync workflow lives in UPSTREAM_SYNC.md.