feat(skills-next): add SDK reference organization

[evanpurkhiser]

Adds the top-level index.md and STRUCTURE.md for the per-SDK reference tree under skills-next/references/sdks/. This is the organizational scaffold for the skills-next SDK references; the individual per-SDK reference sets land in their own PRs.

[sentry]

Bug: The Cloudflare entry in the Quick Lookup table is missing the hono keyword, which will prevent correct detection of projects using Hono on Cloudflare.
_{Severity: MEDIUM}

Suggested Fix

Add the hono keyword to the list of keywords for the Cloudflare entry in the Quick Lookup table at skills-next/references/sdks/index.md:93 to ensure projects using Hono on Cloudflare are correctly identified.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: skills-next/references/sdks/index.md#L93

Potential issue: The Quick Lookup table in `skills-next/references/sdks/index.md` is
missing the `hono` keyword in the Cloudflare entry. This omission prevents projects
using the Hono framework on Cloudflare from being correctly matched to the `cloudflare`
slug during keyword-based detection. This contradicts several authoritative sources
within the repository (`SKILL_TREE.md`, `scripts/build-skill-tree.sh`, and other
documentation) which all include `hono` as a valid keyword for Cloudflare, where it is
described as a first-class use case.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[evanpurkhiser]

Suggested change

	The per-platform HOW — the code for installing Sentry and wiring up each signal. There is one
	The per-platform sentry configuration. the code for installing Sentry and wiring up each different type of instrumentation. There is one

[evanpurkhiser]

I don't think this is totally true, we should be indicating that source maps will likely need to be setup, and will potentially have a source-maps reference document specific to this SDK. same for releases and datascrubbing.

That said we would not want to overexpliain the CONCEPTS of what these are, given the LLM will be pointed to like concepts/source-maps.md or whatever

[evanpurkhiser]

Merging these ahead of review to get the ball rolling on the new skills layout. @cleptric will follow up with SDK teams to finesse the content of the SDK references.