Clarify Tab Pilot release channels

[ResearchForumOnline]

Summary

• remove the stale Gemma-specific planner description
• make the Chrome Web Store the explicit preferred public install
• label the 0.2.0 download and metadata as the managed/offline self-hosted package
• preserve the legacy self-hosted identity without claiming unpublished source version 0.3.0 is in the Store
• add contract coverage for channel wording

Verification

• Tab Pilot contract tests pass
• complete OpenZero suite: 78 passed
• diff check passes