story: Build tab — merge "Build on Celo" and Tooling into build/*, with Agents and Mini Apps leading

[GigaHierz]

Part of #2266.

User story

As an app or agent developer, I want one Build tab ordered Quickstart → Agents → Mini Apps → Network info → Guides → Tools → Reference, so the primary paths are one click from the tab root instead of the third group of the second tab.

Context & evidence

At bdf40b37: "Build on Celo" (docs.json:111) has 31 pages; "Tooling" (docs.json:188) has 90 pages — 31% of all pages in a junk-drawer tab with ten groups. The agent content lives at build-on-celo/build-with-ai/ (10 pages: overview, use-docs-with-ai, 8004, x402, mpp, celopedia, vibe-coding, usecases, mcp/index, mcp/celo-mcp) plus build-on-celo/attribution-tags.mdx. Naming drifts across "Build with AI" / "Build for MiniPay" / "Build on SocialConnect".

Scope

Move build-on-celo/* and the developer subset of tooling/* into build/* per the #2209 Appendix A map at a14395f8926eb43424e0d20e6d67ab96f2c8209b, with these corrections:

• Agents (build-with-ai/* incl. mpp, celopedia, mcp/*, plus attribution-tags) and Mini Apps promoted to top-level groups in the order above
• The map's build-with-ai/agent-skills page does not exist — drop it
• thirdweb is one tool page at parity after task: De-promote thirdweb — keep one tool page at parity with other tools, drop code examples and recommendations #2255 — place it in the tools group, not in Guides
• Mini Apps group holds the single MiniPay overview from story: MiniPay — slim the section to one overview that routes to docs.minipay.xyz #2264
• Wallet integration page from story: Split wallet docs into an end-user page and a developer wallet-integration page #2256; fee-abstraction guide as differentiated in task: Fee abstraction — differentiate the guide, contract-reference and spec pages and make their relationship explicit #2257
• tooling/nodes/run-a-celo-node → Operate (story: Operate tab — move infra-partners/* to operate/* and fold Specs in as a Specification section #2260 owns the destination; this PR only adds the agreed redirect)
• One preposition convention: "Build with X" for tools/SDKs, "Build for X" for platforms; flatten the "Build with AI" sub-groups (Agent Infrastructure / MCP Servers) into one ordered list
• Redirects: catch-alls /build-on-celo/:slug* → /build/:slug* and /tooling/:slug* → /build/tools/:slug* plus per-file overrides

Acceptance criteria

• Tab renamed "Build"; Tooling tab removed
• Group order: Quickstart → Agents → Mini Apps → Network info → Guides → Tools → Reference
• Every page that survives task: Orphaned-pages audit — re-nav or delete the 22 pages on disk that are missing from navigation #2253 / task: De-promote thirdweb — keep one tool page at parity with other tools, drop code examples and recommendations #2255 / story: MiniPay — slim the section to one overview that routes to docs.minipay.xyz #2264 and belongs to Build is present; nothing else
• Redirects verified by curling 20 sampled old URLs → 200 at the new path
• home/celo.mdx cards and llms.txt resolve to the new paths
• npx mintlify broken-links + orphan check green

Non-goals

x402 / first-mile content (#2263); Self Agent ID (#2262); MiniPay content (#2264).

Depends on / blocks

Depends on #2251, #2252, #2253, #2255, #2256, #2257, #2264. Blocks #2261, #2263.

Measured at: bdf40b37