docs: ship Home 7 and agent on existing channels

[wukongnotnull]

Summary

• Refresh the five-language READMEs and Home screenshot so existing channels show the seven-item menu (Worktree + Agent) and list vole agent as a shipped command.
• Spell out that product “v3” is a CLI generation name: package versions stay on 2.x (currently 2.19.1), not 3.0.0, and Homebrew stays tap-only (no homebrew-core PR).
• Align install-assistant prompts and Formula/vole.rb comments with tap-only install and the extra mutating commands (purge / worktree / agent).

This PR is based on feat/v3-t3-agent so the diff is channels/docs/screenshot only, not a duplicate of the agent implementation. Merge after #154 (T3).

Test plan

• Confirm this PR’s Files changed is README / Formula / images/tui/home.png only (no agent implementation files vs T3)
• README.md and zh-CN / zh-TW / ja / ko all mention Home item 7 Agent and vole agent --plan
• images/tui/home.png shows seven items including Agent
• Install prompt forbids Homebrew core and extra mutating commands
• Formula/vole.rb comments say do not open a homebrew-core PR
• Merge only after T3 (feat/v3-t3-agent, feat: add vole agent leftover cleanup command #154) is on main (or retarget to main after T3 merges)

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