docs(contributing): add CONTRIBUTING, code of conduct and issue/PR templates

Add CONTRIBUTING.md covering all validation gates and contribution
workflows, CODE_OF_CONDUCT.md (Contributor Covenant 2.1), GitHub issue
templates (bug/feature), and a pull request template; add a Contributing
section to the README.

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -0,0 +1,66 @@
+name: Bug report
+description: A diagram, script, or import path misbehaves — report it so we can fix it.
+title: "[Bug]: "
+labels: ["bug"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for reporting. Before submitting, check that this isn't an already-open issue.
+  - type: dropdown
+    id: area
+    attributes:
+      label: Area
+      description: What part of the project is affected?
+      options:
+        - A diagram example
+        - Style guide / skin / linter
+        - draw.io import
+        - Mermaid import
+        - Export (SVG / PNG)
+        - Documentation
+        - CI / tooling
+        - Other
+    validations:
+      required: true
+  - type: textarea
+    id: description
+    attributes:
+      label: What happened?
+      description: Describe the unexpected behavior. Include what you expected instead.
+      placeholder: "The linter accepts a diagram that breaks the accessible SVG contract…"
+    validations:
+      required: true
+  - type: textarea
+    id: repro
+    attributes:
+      label: Steps to reproduce
+      description: Concrete steps — commands, file names, and inputs matter here.
+      placeholder: |
+        1. Run `python3 scripts/lint-skin.py <file>`
+        2. …
+    validations:
+      required: true
+  - type: input
+    id: environment
+    attributes:
+      label: Environment
+      description: OS, Python version, agent (Claude Code / Codex / Pi) if relevant
+      placeholder: "macOS 14, Python 3.11, Claude Code"
+  - type: checkboxes
+    id: gates
+    attributes:
+      label: Validation gates
+      description: If relevant, which gates have you already run?
+      options:
+        - label: "`python3 scripts/test-lint-a11y.py`"
+        - label: "`python3 scripts/lint-skin.py --all --baseline`"
+        - label: "`python3 scripts/verify-drawio-import.py`"
+        - label: "`python3 scripts/verify-mermaid-import.py`"
+        - label: "I haven't run any gates"
+  - type: textarea
+    id: logs
+    attributes:
+      label: Relevant output
+      description: Paste error messages or linter findings, trimmed to what matters.
+      render: text

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: true
+contact_links:
+  - name: Security vulnerability
+    url: https://github.com/cathrynlavery/diagram-design/security/advisories/new
+    about: Report a suspected vulnerability privately — do not open a public issue for it.

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,45 @@
+name: Feature request
+description: Suggest a new diagram type, primitive, or improvement.
+title: "[Feature]: "
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Ideas are welcome. Before submitting, check the README's "When *not* to use this skill" section — sometimes the right answer is a table, not a diagram.
+  - type: textarea
+    id: problem
+    attributes:
+      label: What are you trying to show?
+      description: Describe the situation and what the reader needs to understand. A sketch or ASCII mock helps a lot.
+      placeholder: "I want to show how... and the reader should see..."
+    validations:
+      required: true
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposed solution
+      description: How could the skill serve this? A new type, a primitive, a change to an existing one, docs?
+      placeholder: "A new `type-*.md` for …, or an extension of …"
+  - type: dropdown
+    id: scope
+    attributes:
+      label: Scope
+      description: Roughly how much surface does this touch?
+      options:
+        - New diagram type (reference + 3 examples + gallery)
+        - New or changed primitive
+        - Import grammar support
+        - Tooling / scripts / CI
+        - Documentation only
+        - Not sure yet — help me scope it
+    validations:
+      required: true
+  - type: checkboxes
+    id: philosophy
+    attributes:
+      label: Design-system fit
+      description: Features in this project must fit the editorial philosophy in SKILL.md §1–§2.
+      options:
+        - label: "I've checked the anti-patterns and 'When not to use' — this still seems worth drawing"
+        - label: "I'm willing to contribute the implementation or pair with a maintainer"

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,26 @@
+## What does this PR do?
+
+<!-- One or two sentences: what changed and why. -->
+
+## Validation gates
+
+<!-- Mark what you ran and that it passed. CI runs all of these on the PR too. -->
+
+- [ ] `python3 scripts/test-lint-a11y.py`
+- [ ] `python3 scripts/lint-skin.py --all --baseline`
+- [ ] `python3 scripts/verify-sequence-oauth.py`
+- [ ] `python3 scripts/verify-drawio-import.py`
+- [ ] `python3 scripts/verify-mermaid-import.py`
+- [ ] `git diff --exit-code` green after `python3 scripts/build-icons.py` (if icons changed)
+- [ ] Docs updated in the same PR where behavior changed
+
+## Checklist
+
+- [ ] No new entries added to `scripts/lint-skin-baseline.txt`
+- [ ] Generated and source files are consistent (extractor ↔ verifier ↔ reference ↔ command)
+- [ ] Accessible SVG contract satisfied for new/changed examples
+- [ ] Code of Conduct respected
+
+## Related issues
+
+<!-- Link any issues this closes, e.g. "Closes #12". -->