chore: add dependabot and issue and pull request templates

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: nos1dot618

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Problem with the language or lbd
+    url: https://github.com/lambda-discipline/lambda-discipline/issues/new/choose
+    about: Bugs and proposals for the language itself belong in the language repository.
+  - name: Report a security vulnerability
+    url: https://github.com/lambda-discipline/lambda-discipline.github.io/security/advisories/new
+    about: Please report vulnerabilities privately, not as public issues.

.github/ISSUE_TEMPLATE/site_problem.yml

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+name: Problem on the site
+description: A broken page or link, a mistake in the docs, or something that looks wrong.
+labels: [bug]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for reporting! Problems with the language or `lbd` itself belong in the
+        [language repository](https://github.com/lambda-discipline/lambda-discipline/issues).
+  - type: input
+    id: url
+    attributes:
+      label: Page
+      placeholder: https://lambda-discipline.github.io/reference/#types
+    validations:
+      required: true
+  - type: dropdown
+    id: kind
+    attributes:
+      label: Kind of problem
+      options:
+        - Broken link
+        - Wrong or unclear documentation
+        - Code sample doesn't work
+        - Layout or display
+        - Other
+    validations:
+      required: true
+  - type: textarea
+    id: problem
+    attributes:
+      label: What's wrong
+      description: What you saw, and what you expected instead.
+    validations:
+      required: true
+  - type: input
+    id: browser
+    attributes:
+      label: Browser and device
+      description: Only needed for layout problems.
+      placeholder: Firefox 140 on Android

.github/ISSUE_TEMPLATE/suggestion.yml

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+name: Suggestion
+description: A new page, example, explanation or design improvement.
+labels: [enhancement]
+body:
+  - type: textarea
+    id: idea
+    attributes:
+      label: Idea
+      description: What should the site have, and who would it help?
+    validations:
+      required: true
+  - type: textarea
+    id: details
+    attributes:
+      label: Details
+      description: Draft text, an example program, a sketch, or sites that do it well.

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: pip
+    directory: /
+    schedule:
+      interval: weekly
+    groups:
+      # Patch and minor bumps arrive as one PR; major versions get their own.
+      pip-minor:
+        update-types: [minor, patch]
+    commit-message:
+      prefix: deps
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    groups:
+      actions:
+        patterns: ["*"]
+    commit-message:
+      prefix: ci

.github/pull_request_template.md

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+## Summary
+
+<!-- What changes on the site, and why. Link the issue: "Closes #123". -->
+
+## Checklist
+
+- [ ] `black --check .`, `ruff check .` and `mypy` pass
+- [ ] `python build.py` succeeds and `lychee --offline --root-dir "$PWD/_site" _site` finds no broken links
+- [ ] I looked at the changed pages in light and dark mode, and at phone width
+- [ ] Code samples run with `lbd run`, and their comments show what they really print