Add CI release workflow, CHANGELOG.md, issue templates

- .github/workflows/release.yml: on tag push, build exe via PyInstaller,
  package portable zip, attach SHA-256, create GitHub Release.
- CHANGELOG.md: Keep a Changelog format, semver.
- .github/ISSUE_TEMPLATE/: bug_report.yml + feature_request.yml + config.yml
  routing security reports to private advisories.

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -0,0 +1,56 @@
+name: Bug report
+description: Something doesn't work as expected
+labels: ["bug"]
+body:
+  - type: input
+    id: version
+    attributes:
+      label: Version
+      description: Visible in the startup banner.
+      placeholder: v1.1.0
+    validations:
+      required: true
+
+  - type: textarea
+    id: steps
+    attributes:
+      label: Steps to reproduce
+      placeholder: |
+        1. ...
+        2. ...
+        3. ...
+    validations:
+      required: true
+
+  - type: textarea
+    id: expected
+    attributes:
+      label: What you expected to happen
+    validations:
+      required: true
+
+  - type: textarea
+    id: actual
+    attributes:
+      label: What actually happened
+      description: Paste any error output verbatim. Screenshots are welcome.
+    validations:
+      required: true
+
+  - type: input
+    id: os
+    attributes:
+      label: Windows version
+      placeholder: e.g. Windows 11 24H2
+
+  - type: input
+    id: network
+    attributes:
+      label: Network setup (if relevant)
+      placeholder: e.g. USB Realtek NIC to a server's BMC port via an 8-port switch
+
+  - type: textarea
+    id: extra
+    attributes:
+      label: Anything else?
+      description: Workarounds tried, related links, etc.

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Security vulnerability
+    url: https://github.com/Engelgardt23/dhcpsrv/security/advisories/new
+    about: Please report security issues privately via GitHub Security Advisories — not as a public issue.

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,23 @@
+name: Feature request
+description: Suggest a new feature or an improvement
+labels: ["enhancement"]
+body:
+  - type: textarea
+    id: motivation
+    attributes:
+      label: What's the use case?
+      description: What are you trying to do, and why is the current behavior not enough?
+    validations:
+      required: true
+
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposed solution
+    validations:
+      required: true
+
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered