docs: add typed contribution templates and tighten contributor guidelines (#1673)

Overhaul CONTRIBUTING.md and all GitHub issue/PR templates to enforce a
structured, approval-gated contribution process that cuts down on drive-by
feature submissions.

Changes:
- CONTRIBUTING.md: add Types of Contributions section defining Fix,
  Enhancement, and Feature with escalating requirements and explicit
  rejection criteria; add Issue-First Rule section making clear that
  enhancements require approved-enhancement and features require
  approved-feature label before any code is written; backport gsd-2
  testing standards (t.after() per-test cleanup, array join() fixture
  pattern, Node 24 as primary CI target, test requirements by change type,
  reviewer standards)

- .github/ISSUE_TEMPLATE/enhancement.yml: new template requiring current
  vs. proposed behavior, reason/benefit narrative, full scope of changes,
  and breaking changes assessment; cannot be clicked through

- .github/ISSUE_TEMPLATE/feature_request.yml: full rewrite requiring solo-
  developer problem statement, what is being added, full file-level scope,
  user stories, acceptance criteria, maintenance burden assessment, and
  alternatives considered; incomplete specs are closed, not revised

- .github/pull_request_template.md: converted from general template to a
  routing page directing contributors to the correct typed template;
  using the default template for a feature or enhancement is a rejection
  reason

- .github/PULL_REQUEST_TEMPLATE/fix.md: new typed template requiring
  confirmed-bug label on linked issue and regression test confirmation

- .github/PULL_REQUEST_TEMPLATE/enhancement.md: new typed template with
  hard gate on approved-enhancement label and scope confirmation section

- .github/PULL_REQUEST_TEMPLATE/feature.md: new typed template requiring
  file inventory, spec compliance checklist from the issue, and scope
  confirmation that nothing beyond the approved spec was added

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE/enhancement.md

@@ -0,0 +1,86 @@
+## Enhancement PR
+
+> **Using the wrong template?**
+> — Bug fix: use [fix.md](?template=fix.md)
+> — New feature: use [feature.md](?template=feature.md)
+
+---
+
+## Linked Issue
+
+> **Required.** This PR will be auto-closed if no valid issue link is found.
+> The linked issue **must** have the `approved-enhancement` label. If it does not, this PR will be closed without review.
+
+Closes #
+
+> ⛔ **No `approved-enhancement` label on the issue = immediate close.**
+> Do not open this PR if a maintainer has not yet approved the enhancement proposal.
+
+---
+
+## What this enhancement improves
+
+<!-- Name the specific command, workflow, or behavior being improved. -->
+
+## Before / After
+
+**Before:**
+<!-- Describe or show the current behavior. Include example output if applicable. -->
+
+**After:**
+<!-- Describe or show the behavior after this enhancement. Include example output if applicable. -->
+
+## How it was implemented
+
+<!-- Brief description of the approach. Point to the key files changed. -->
+
+## Testing
+
+### How I verified the enhancement works
+
+<!-- Manual steps or automated tests. -->
+
+### Platforms tested
+
+- [ ] macOS
+- [ ] Windows (including backslash path handling)
+- [ ] Linux
+- [ ] N/A (not platform-specific)
+
+### Runtimes tested
+
+- [ ] Claude Code
+- [ ] Gemini CLI
+- [ ] OpenCode
+- [ ] Other: ___
+- [ ] N/A (not runtime-specific)
+
+---
+
+## Scope confirmation
+
+<!-- Confirm the implementation matches the approved proposal. -->
+
+- [ ] The implementation matches the scope approved in the linked issue — no additions or removals
+- [ ] If scope changed during implementation, I updated the issue and got re-approval before continuing
+
+---
+
+## Checklist
+
+- [ ] Issue linked above with `Closes #NNN` — **PR will be auto-closed if missing**
+- [ ] Linked issue has the `approved-enhancement` label — **PR will be closed if missing**
+- [ ] Changes are scoped to the approved enhancement — nothing extra included
+- [ ] All existing tests pass (`npm test`)
+- [ ] New or updated tests cover the enhanced behavior
+- [ ] CHANGELOG.md updated
+- [ ] Documentation updated if behavior or output changed
+- [ ] No unnecessary dependencies added
+
+## Breaking changes
+
+<!-- Does this enhancement change any existing behavior, output format, or API?
+     If yes, describe exactly what changes and confirm backward compatibility.
+     Write "None" if not applicable. -->
+
+None