chore: add workflow to self test the action

[KROSF]

Summary

• Add .github/workflows/self-test.yml, which runs the action from the checkout (uses: ./) against fixture charts in __tests__/fixtures/:
  • changed: charts differ by one line → asserts changed=true, the diff contains the added line as markdown, and targetTemplate is populated.
  • unchanged: same chart on both sides → asserts changed=false and an empty diff.
• Add an optional targetValuesBranch input (default develop). Previously the target values ref was hardcoded to develop unless the PR body matched targetBranchRegex, so the action couldn't be self-tested in this repo (no develop branch). Existing behavior is unchanged.
• Rebuild dist/.

Test plan

• Fixtures render with helm template locally and produce the expected diff
• pnpm lint, pnpm format-check, pnpm test pass
• self-test workflow passes on this PR

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