Android: fix remote skill URL validation for GitHub URLs

[txhno]

Partial fix for #619 on the Android client.

This improves remote skill import in four ways:

• normalize common GitHub tree/blob/raw URLs into a fetchable SKILL.md path
• detect HTML responses early and return a clearer validation error
• reject run_js skills hosted on github.com / raw.githubusercontent.com with an explicit hosting message instead of a misleading frontmatter parse failure
• document the supported GitHub-hosted URL forms and clarify that this repository currently contains the Android-side implementation of that URL-loading flow

The issue reporter also mentioned iOS/macOS behavior, but I do not see the corresponding iOS/macOS client source in this repository to patch here. If that client code is available elsewhere, the same normalization / raw-content validation approach can be mirrored there.

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[tg-agent]

Code review

Found 1 issue:

1. The PR fixes issue [Bug]: Can't install custom or example skills - failing with same error message #619, but the issue was filed from an iPhone 15 Pro running iOS — and the reporter explicitly noted the same failure on both iPhone and Mac. All changes in this PR target the Android Kotlin source files; no iOS counterpart has been made available in this repo.

gallery/Android/src/app/src/main/java/com/google/ai/edge/gallery/customtasks/agentchat/SkillManagerViewModel.kt

Lines 1 to 5 in b33094b

	/*
	* Copyright 2026 Google LLC
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.

The Android-side URL normalization and HTML-detection improvements look correct, but the linked issue will not be resolved for iOS users.

Recommendation for maintainers: merge this fix into the Android app source and comment on how it can be ported into the iOS version of the app.

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[txhno]

Pushed a follow-up commit on this branch (3c5a61b) that makes the repo docs explicit about scope: the URL-loading behavior documented and implemented here is the Android client path in this repository.

[txhno]

@tg-agent review