Add X Chat read/reply skill for the X plugin

[vaulpannx]

Summary

• Add `skills/x-chat/SKILL.md`: clone xchat-grokbot-helper, secret-request Chat PIN only, decrypt/encrypt locally with `xchat_lite.py`, send ciphertext via MCP. Owner must approve outbound text unless they already said to send/reply.
• Update the X MCP guide: Chat on-connect capability; missing Chat tools or `dm.read`/`dm.write` while other X tools work → reconnect the X plugin (not create a Project/App, not a Bearer token).
• Bump plugin to 2.5.0.

Test plan

• Connect X with Chat tools present: capabilities include Chat; reading inbox uses helper + PIN via secret-request (PIN never pasted in chat).
• Other X tools work but Chat tools missing / 403 on dm scopes: quoted reauth line, not account-not-ready Project/App copy.
• Send: ask for owner approval unless they already said to reply/send; `session-encrypt` then `send_chat_message`.
• Do not ask for Bearer tokens or sign in via the agent browser.

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