Show agent provider icons in the thread list, with color options

[mistakeknot]

Human comments

What was wrong

The sidebar thread list gives no hint of which agent provider a thread runs on. For someone running threads on several providers (for example, moving a long-lived thread between Claude Code and Codex as quota runs out), the only way to tell is to open each thread. The mobile recents list already draws a provider tile, and the sidebar list had no equivalent. See #4088.

What changed

• plugins/thread-list/app/rows/ThreadRow.tsx: a ThreadRowProviderIcon leads the title. It looks the row's providerId up in experimental_useProviders() and draws experimental_ProviderIcon (providerKind="agent"), labelled and titled with the provider's displayName. It draws nothing until the directory knows the id, or when the preference is off.
• plugins/thread-list/shared/preferences.ts / app/preferences/atoms.ts: new synced showProviderIcons preference (boolean, default true, no legacy key).
• plugins/thread-list/app/list/SidebarViewItems.tsx: Organize gains a Rows group with a Provider icons checkbox, beside the existing Groups → By environment item.
• Icon color (second commit): app/rows/provider-icon-color.ts resolves each icon's color from, in order, a custom per-provider light/dark color (providerIconColors), the theme's --provider-icon-<id> then --provider-icon CSS variables, and the providerIconColor mode, brand (the provider's own iconTint, default) or monochrome (row text color). Organize → Rows adds Brand colors / Monochrome and a Customize colors… dialog (app/list/ProviderIconColorsDialog.tsx) with light and dark pickers per provider. Colors are validated with isPresentationTintColor in the preference schema and again before use.
• Docs: docs/configuration.md (new "Thread list provider icons" subsection, including color precedence), theming.md (the two theme variables) and the thread-list skill's key list. bb thread-list prefs set showProviderIcons false works through the existing generic prefs CLI.
• No wire, protocol, or core app changes.

How you verified

• New tests in plugins/thread-list/app/rows/ThreadRow.test.tsx cover a known provider drawing its icon with the provider's label, the preference hiding it, and an unknown provider id drawing nothing. SidebarHeaderControls.test.tsx covers the Rows → Provider icons toggle writing the preference both ways.
• provider-icon-color.test.ts, ProviderIconColorsDialog.test.tsx and shared/preferences.test.ts cover the color precedence, the dialog's defaults, partial edits and reset, and schema rejection of non-color values.
• npx vitest run in plugins/thread-list: 29 files, 326 tests passed. tsc --noEmit in plugins/thread-list: clean.
• An earlier version of the same row change on the pre-Delete the built-in thread list and show the placeholder while the plugin loads #4041 built-in list ran on a live 0.43.3 install. This plugin version has only been exercised in tests, not in a browser.

Fixes #4088

AGENT GENERATED