fix(3696): pr-template enforcer recognises CI/tooling carve-out (#3697)

* fix(3696): pr-template enforcer recognises CI/tooling carve-out

The enforcer in scripts/pr-template-policy.cjs only validated against
three typed templates and three hard-coded DEFAULT_TEMPLATE_MARKERS.
It had zero awareness of the documented CI/tooling/dep/doc-only exception
in .github/pull_request_template.md, meaning external contributors who
followed the documented escape hatch still had their PRs auto-closed.

Fix:
1. Path-scope auto-skip: if every changed file matches a tooling glob
   allowlist (.github/**, scripts/**, docs/**, *.md, .changeset/**,
   dependency manifests), skip enforcement and exit success with no
   comment posted.
2. Explicit exemption marker: if the PR body contains
   <!-- pr-template-exempt: <non-empty reason> -->, skip enforcement.
3. Workflow updated to fetch changed file paths via gh and pass them
   as CHANGED_FILES env to the policy script.
4. Pull request template updated to document both mechanisms and retire
   the old "delete this file" prose carve-out.
5. 14 new tests (TDD red→green); all 25 tests pass.

Closes #3696

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(3696): allow hyphenated pr-template exemption reasons

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