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

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 #2255 — place it in the tools group, not in Guides
- Mini Apps group holds the single MiniPay overview from #2264
- Wallet integration page from #2256; fee-abstraction guide as differentiated in #2257
- `tooling/nodes/run-a-celo-node` → Operate (#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 #2253 / #2255 / #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`