docs: add SDK cleanup checklist and PR template

[euniceamoni]

Summary

Adds contributor cleanup standards so every PR can be held to a consistent bar before review.

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Type of change

• Documentation only

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What's included

• docs/CLEANUP_CHECKLIST.md — 8-section reference document covering:

  • Module boundaries (barrel imports, no deep cross-module imports, circular dependency prevention)
  • Naming conventions (files, classes, errors, booleans, constants)
  • Import/export rules (explicit barrels, no wildcard re-exports, peer dependency handling)
  • Testing expectations (happy + error paths, file placement, approved mock helpers, no test.only)
  • Error and config conventions (AxionveraError subclasses, no hardcoded URLs, secrets never logged)
  • Documentation standards (TSDoc, docs/ updates, working examples)
  • Code quality (lint, typecheck, build, size, no console.log in library code)
  • Monorepo hygiene (package READMEs, shim exports, independent versioning)

• .github/PULL_REQUEST_TEMPLATE.md — PR template with a condensed checkbox version of the checklist, plus standard metadata fields (summary, change type, testing, breaking changes).

• README.md — Table of contents entry and a new ## 🧹 Cleanup Checklist section linking to both documents.

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Testing

Documentation-only change. No source code modified. closes #325