chore: add release checks and docs

[jerryfane]

What

Adds release readiness documentation, a CI workflow, manifest validation, and an env template with variable names only.

Why

Task 4 requires public docs polish and a repeatable verification path before the first Agentgram release.

Changes

• Added .env.example with empty Telegram variable names only.
• Added GitHub Actions CI for unit tests, manifest validation, and committed whitespace checks.
• Added scripts/validate_manifest.py for plugin/env/docs validation.
• Added docs/release-checklist.md with fresh-clone and optional live Telegram smoke steps.
• Expanded README troubleshooting for bot not started, bad token, missing chat id, forbidden chat, and Telegram API errors.

Verification

• python3 -m unittest discover -s tests -v passed locally: 32 tests.
• python3 scripts/validate_manifest.py passed locally.
• python3 /root/.codex/skills/.system/plugin-creator/scripts/validate_plugin.py /root/agentgram passed locally.
• git diff --check passed locally.
• Fresh clone smoke from /root/agentgram at 48bcf45 passed: tests, manifest validation, bin/agentgram --help, and expected clear doctor failure without Telegram env vars.
• Live Telegram smoke not run; no test credentials were provided.

Review

codex exec review --uncommitted: No actionable correctness issues were found in the staged, unstaged, or untracked changes. The added CI, validation script, env example, and release documentation appear consistent with the current repository layout.

Risk

Low. This is docs and release validation only; no runtime send behavior changed.