Check X credit balance before telling users to buy

[vaulpannx]

Summary

• Follow-up to Document X credits endpoint in the MCP guide #254: agents must call get_usage_credits before any user-facing X text on connect.
• Ban the legacy “you’ll need to purchase credits at https://console.x.com” line unless the check returned ~$0 or the planned job would exceed the balance.
• On-connect copy now states the dollar balance and ideas; console.x.com is mentioned only when they actually need to add credits.

Test plan

• Connect X with a non-zero balance: capabilities message includes the amount and does not tell the user to buy credits or visit console.x.com.
• Connect X with ~$0: after the credits fetch, then point them to https://console.x.com.
• Confirm the agent never greets / lists capabilities / mentions purchasing until get_usage_credits has returned.

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