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/feature.md

@@ -0,0 +1,113 @@
+## Feature PR
+
+> **Using the wrong template?**
+> — Bug fix: use [fix.md](?template=fix.md)
+> — Enhancement to existing behavior: use [enhancement.md](?template=enhancement.md)
+
+---
+
+## Linked Issue
+
+> **Required.** This PR will be auto-closed if no valid issue link is found.
+> The linked issue **must** have the `approved-feature` label. If it does not, this PR will be closed without review — no exceptions.
+
+Closes #
+
+> ⛔ **No `approved-feature` label on the issue = immediate close.**
+> Do not open this PR if a maintainer has not yet approved the feature spec.
+> Do not open this PR if you wrote code before the issue was approved.
+
+---
+
+## Feature summary
+
+<!-- One paragraph. What does this feature add? Assume the reviewer has read the issue spec. -->
+
+## What changed
+
+### New files
+
+<!-- List every new file added and its purpose. -->
+
+| File | Purpose |
+|------|---------|
+| | |
+
+### Modified files
+
+<!-- List every existing file modified and what changed in it. -->
+
+| File | What changed |
+|------|-------------|
+| | |
+
+## Implementation notes
+
+<!-- Describe any decisions made during implementation that were not specified in the issue.
+     If any part of the implementation differs from the approved spec, explain why. -->
+
+## Spec compliance
+
+<!-- For each acceptance criterion in the linked issue, confirm it is met. Copy them here and check them off. -->
+
+- [ ] <!-- Acceptance criterion 1 from issue -->
+- [ ] <!-- Acceptance criterion 2 from issue -->
+- [ ] <!-- Add all criteria from the issue -->
+
+## Testing
+
+### Test coverage
+
+<!-- Describe what is tested and where. New features require new tests — no exceptions. -->
+
+### Platforms tested
+
+- [ ] macOS
+- [ ] Windows (including backslash path handling)
+- [ ] Linux
+
+### Runtimes tested
+
+- [ ] Claude Code
+- [ ] Gemini CLI
+- [ ] OpenCode
+- [ ] Codex
+- [ ] Copilot
+- [ ] Other: ___
+- [ ] N/A — specify which runtimes are supported and why others are excluded
+
+---
+
+## Scope confirmation
+
+- [ ] The implementation matches the scope approved in the linked issue exactly
+- [ ] No additional features, commands, or behaviors were added beyond what was approved
+- [ ] If scope changed during implementation, I updated the issue spec and received re-approval
+
+---
+
+## Checklist
+
+- [ ] Issue linked above with `Closes #NNN` — **PR will be auto-closed if missing**
+- [ ] Linked issue has the `approved-feature` label — **PR will be closed if missing**
+- [ ] All acceptance criteria from the issue are met (listed above)
+- [ ] Implementation scope matches the approved spec exactly
+- [ ] All existing tests pass (`npm test`)
+- [ ] New tests cover the happy path, error cases, and edge cases
+- [ ] CHANGELOG.md updated with a user-facing description of the feature
+- [ ] Documentation updated — commands, workflows, references, README if applicable
+- [ ] No unnecessary external dependencies added
+- [ ] Works on Windows (backslash paths handled)
+
+## Breaking changes
+
+<!-- Describe any behavior, output format, file schema, or API changes that affect existing users.
+     For each breaking change, describe the migration path.
+     Write "None" only if you are certain. -->
+
+None
+
+## Screenshots / recordings
+
+<!-- If this feature has any visual output or changes the user experience, include before/after screenshots
+     or a short recording. Delete this section if not applicable. -->