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.

.github/workflows/build.yml

@@ -0,0 +1,49 @@
+name: Build
+
+# Verifies that every push to main and every PR still builds against the
+# current Dalamud staging branch. Does not produce release artefacts; the
+# release workflow handles that on tag.
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  build:
+    name: Build (Release)
+    runs-on: windows-latest
+    timeout-minutes: 15
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup .NET 10
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: 10.0.x
+
+      - name: Download Dalamud staging
+        shell: pwsh
+        run: |
+          $hooks = Join-Path $env:APPDATA "XIVLauncher\addon\Hooks\dev"
+          New-Item -ItemType Directory -Force -Path $hooks | Out-Null
+          Invoke-WebRequest -Uri https://goatcorp.github.io/dalamud-distrib/stg/latest.zip -OutFile dalamud.zip
+          Expand-Archive -Force -Path dalamud.zip -DestinationPath $hooks
+
+      - name: Restore
+        run: dotnet restore ChatTwo/ChatTwo.csproj
+
+      - name: Build (Release)
+        run: dotnet build ChatTwo/ChatTwo.csproj --configuration Release --no-restore
+
+      - name: Upload build output
+        uses: actions/upload-artifact@v4
+        with:
+          name: HellionChat-build-${{ github.run_number }}
+          path: ChatTwo/bin/Release/**/HellionChat/**
+          if-no-files-found: warn
+          retention-days: 14