feat: add SDK 9.x migration support to the LevelPlay skill

[ziyiunity]

Adds guided SDK 9.x migration to the LevelPlay skill, so a developer arriving with an existing IronSource or older LevelPlay integration gets a path forward instead of instructions written for a fresh install.

What lands

references/migration-sdk-9.md (666 lines) covers five scenarios: SDK upgrade by .unitypackage or UPM including switching between the two, init API migration off IronSource.Agent, ad unit API migration per format, Maven Central dependency build failures, and Unity Ads migration. It ends with a completeness checklist for the requirements a line-by-line translation misses, because the legacy code had no equivalent line to translate: placement capping checks, an explicit rewarded load trigger, correct ILRD event names, HideAd versus DestroyAd intent for legacy destroyBanner calls, and removal of onApplicationPause.

SKILL.md gains a Step 0 that asks whether this is a new integration, a migration, or troubleshooting, before any other question. Getting that wrong early is expensive: a migration walked through the new-integration path ends up with a second parallel setup rather than a converted one.

Also in SKILL.md: a troubleshooting row for Android builds that suddenly fail resolving com.ironsource.sdk from android-sdk.is.com, which is the signature of that repository being shut down and dependencies having moved to Maven Central; a pointer from the deprecated-API branch to the new reference; and the placement-capping requirement added to the code-generation rules.

Five reference files take corresponding updates: banner-api.md, best-practices.md, initialization-api.md, interstitial-api.md, privacy-settings.md. The banner fix is worth naming: the adaptive-size example used a constructor form that does not compile on any 9.x version, and is now configured through Config.Builder.

How SKILL.md was edited

Surgically, four insertions, rather than replaced wholesale. The install-verification gate at Step 3 and the CS0246 material in references/troubleshooting.md are unchanged and were not in scope here.

Step 0 asks the user to check the Unity console for compilation errors rather than naming a tool to run. No skill in this set invokes a CLI or drives the Editor, and a step that names a mechanism the agent cannot reach is worse than a step that names none: the agent will try it, fail, and either stall or invent a substitute. It is also written not to block when the console cannot be seen, so the run ends with a list of changed files and what to look for.

Checked

• Structural gate passes: frontmatter, description under the listing cap, no author field, no tool names that do not exist here.
• All twelve references/*.md mentioned in the body exist, and every file present is reachable from the body. No orphans in either direction.
• references/migration-sdk-9.md is byte-identical to the copy in Unity-Technologies/skills.
• Each of the five wholesale-updated files was byte-identical to its previous state before replacement, checked per file, so no local edit was overwritten.
• No line-ending or encoding drift. best-practices.md is the largest churn in the diff and it is a genuine reordering, not a whitespace artifact: 604 lines, no CRLF.