fix(init): make the scaffolded starter find the runtime and explain when it can't

[AbirAbbas]

Summary

Follow-up to #216, from the next thing the reporter hit. af-stack init my-ai-product then npm start printed "Could not reach the backend" followed by a raw AgentField 404 body. Two causes, both in the scaffolded starter:

• It assumed the runtime is at localhost:8080, and could not tell "nothing there" from "something else there". af-stack dev deliberately moves the API off 8080 whenever the port is busy (on the reporter's machine 8080 is an AgentField control plane), but the starter never said where the real URL comes from.
• npm start never read .env. It ran plain node src/index.mjs, so the documented cp .env.example .env and "set AF_STACK_URL" had no effect at all.

Changes

• The starter loads .env itself with no dependency (real environment wins), tolerates a pasted …/api/v1 suffix, and probes /health before its first call. It now distinguishes three failures and gives the fix for each: nothing listening, something listening that is not a BackAI runtime, and an auth rejection. Each points at the "API runtime" line of af-stack dev's output as the source of the URL.
• The scaffold's next steps, README, and .env.example say the same thing.
• Both af-stack dev banners print Your apps AF_STACK_URL=http://localhost:<port>, the exact line to copy, since the "API runtime" line ends in /api/v1 and is not what an app wants.

Verification

• Six tests drive the real src/index.mjs with Node against fake backends: a BackAI runtime (lists agents, exit 0), .env honoured, /api/v1 suffix tolerated, a foreign server on the port (exit 1, names the cause and the fix), nothing listening (exit 1), and a 401 (names AF_STACK_API_KEY). They skip when node is absent.
• Replayed the reporter's exact sequence against this machine's real port 8080, which is held by an AgentField container: the starter now prints "Something is listening at http://localhost:8080, but it is not a BackAI runtime" with the af-stack dev / AF_STACK_URL instructions.
• Full go build, go vet, go test ./... pass; golangci-lint v2.13.1 in CI's new-issues mode reports 0 new issues.

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