Refresh Telegraph maintenance workflow

[alterxyz]

Summary

• Add deterministic unit tests for Markdown conversion, API payloads, token loading, and page behavior.
• Keep pip/setuptools as the primary workflow while adding uv compatibility checks.
• Compare against the official Telegraph API docs and update practical gaps: request timeouts, TELEGRA_PH_TOKEN, safer token output, get_page response fields, and delete_page verification.
• Update README, CHANGELOG, AGENTS.md, examples, and CI coverage for Python 3.8-3.14 plus uv.

Verification

• uv run --with-requirements requirements.txt python -m unittest discover -s tests
• uv run --with-requirements requirements.txt python -m compileall ytelegraph tests
• uv run --with build --with-requirements requirements.txt python -m build
• pip venv editable install + unittest + compileall
• uv interpreter sweep for Python 3.9, 3.10, 3.11, 3.12, 3.13, and 3.14
• pip-audit -r requirements.txt found no known vulnerabilities
• Live Telegraph smoke test: create/get/views/list/edit/delete replacement with a disposable temp token

No PyPI release is included in this PR.