plugin: give the plugin its own folder

[erfnzdeh]

#32 made the repository root the plugin. That works, but everyone who installs it receives the whole repository (source, tests, brand images, the demo MP4) to get one Markdown file. It also fails Anthropic's directory rules for a plugin folder: four files over 256 KiB that aren't images (docs/design/command-tree.json, docs/design/parity-catalog.json, tests/fake_telethon.py, tlgr/data/catalog_index.json) and a non-image binary (the MP4) would each hold every version for a manual review.

This moves the plugin into plugin/:

• plugin/.claude-plugin/plugin.json, moved from the root
• plugin/skills/tlgr/SKILL.md, moved from skills/tlgr/SKILL.md (same file, no copy to keep in sync)
• plugin/README.md, new: the directory shows it as the listing and requires 40+ words. It says what the plugin runs and sends: instructions only, no hooks, MCP servers or scripts; Claude runs tlgr commands with the usual prompts; nothing leaves the machine except tlgr's own traffic to Telegram, and a webhook only if you set one.

The root .claude-plugin/marketplace.json now points at ./plugin, so the install commands are unchanged. npx skills add tlgrcli/tlgr still finds the skill at the new path (checked with --list), and the root README link is updated.

Tested: claude plugin validate passes for the marketplace and for ./plugin (the only warning is the deliberately missing version), and claude plugin install tlgr@tlgr from this branch loads Skills (1) tlgr.