feat(tui): filter sidebar to active agents

[Sayt-0]

Summary

• add an Active agents only option under /settings > Appearance > Sidebar sections
• filter vertical and horizontal agent rosters to agents participating in the current session
• persist the opt-in setting as settings.layout.active_agents_only
• preserve full-team agent cycling and original Ctrl+number shortcuts

Behavior

An agent remains visible when it is:

• currently selected or working
• participating in an in-flight transfer or return
• represented by live usage or attributed cost
• represented by attributed cost restored from a reloaded session

The filter is disabled by default. Hiding the Agents section disables the nested option without clearing its saved value.

Validation

• task build
• task lint
• env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy task test
• go test ./pkg/tui/... ./pkg/userconfig/...
• npx --yes markdownlint-cli2@0.22.1 configuration/user-settings/index.md features/tui/index.md from docs/