[Feature] Skill catalog, shared Gradle verification, Navigation 3 workflow

## What is the gap in the LLM's knowledge?
Agents receive conflicting Gradle verification guidance across skills (some require `clean assembleDebug`, others forbid `clean`). The `android-cli` skill embeds a static `android help` dump that goes stale. CameraX skill uses placeholder versions. Navigation 3 skill is index-only without a workflow entry point.

## Proposed Skill
Not a new skill — documentation improvements:

- Shared `GRADLE_VERIFICATION.md` referenced by migration skills
- `SKILLS.md` catalog at repo root
- Navigation 3 "when to use" table
- CameraX version resolution from Maven metadata
- Trim stale CLI help from `android-cli`

## Additional Context
Fork PR: https://github.com/saadkhalidkhan/android-skills/pull/2 (or compare branch `docs/gradle-verification-and-catalog`)

Start by reviewing the proposed changes on the docs/gradle-verification-and-catalog branch or fork PR. Check the requested GRADLE_VERIFICATION.md, root SKILLS.md, Navigation 3 table, CameraX version resolution, and android-cli cleanup; done means all five documentation improvements are consistent and referenced where needed.

Written by the indexing model from the issue text.