Fix wrong claims and style drift on the Celo landing page and Celopedia page #2386

These problems turned up in a review of #2385. That PR didn't cause them, but it touched learn/celo.mdx, and the cards there are the first thing people see on the Celo landing page.

Factual errors

• learn/celo.mdx:8-9: the "Start Building" card says "No coding experience required", but it links to /build/agents/celopedia, where the first step is installing an AI coding skill from a terminal.
• learn/celo.mdx:37: the "Local Stablecoins" card says Mento has "25+ local digital currencies". build/build-with-local-stablecoin.mdx:23 says Mento issues 15 stablecoins, and so does the AGENTS.md section 7 table.
• docs.json:57: the banner says the Agents on Open Rails hackathon "kicks off Oct 6". That date has passed.

AGENTS.md gaps

• docs: replace every rocket card icon #2385 says it applies the no-rocket rule to card icons as well, but AGENTS.md:84 still bans the rocket only in banners. One line in section 6 would cover cards.

Style (AGENTS.md sections 3, 4 and 8)

• learn/celo.mdx:3: the description says "leading", has two sentences and ends with a period.
• learn/celo.mdx: three cards point to Celopedia. "Start Building" (line 8) and "Quickstart" (line 91) link to /build/agents/celopedia, while "Celopedia: Celo Skill" (line 26) links to https://celopedia.celo.org/. The Quickstart card's "under 5 minutes" has no source.
• learn/celo.mdx: the headings at lines 14, 31, 48, 68 and 134 are in title case.
• build/agents/celopedia.mdx:4: the description is promotional, names no task, and ends with a period.
• build/agents/celopedia.mdx:63: ## Resources is a bullet list rather than the Resource | Link table, and the page has no closing ## Related section.