Custom Hex Colors for Named Session Groups

Problem

Named session groups currently share the six-value color enum used by quick session color tags. The daemon rejects any other value with invalid_group_color, the TypeScript SDK exposes the same closed union, and the WebShell editor only offers a preset select. Users cannot align named groups with an existing project palette or visually distinguish a larger group catalog.

Tracked by #6744.

Proposed changes

Layer	Change
Core	Split preset session-tag colors from named-group display colors. Named groups accept presets or six-digit #RRGGBB; quick tags remain preset-only. Normalize valid Hex values to lowercase before persistence.
REST and ACP	Keep quick-tag validation preset-only and pass named-group colors to core validation.
TypeScript SDK	Export preset and Hex color types. Group input/output uses their union; session organization continues to use preset colors.
WebShell	Keep preset choices and add a Custom option with a native color picker and Hex text field. Render custom group dots with an inline background color.

Decisions

• Accept only six-digit #RRGGBB. Three-, four-, and eight-digit forms are rejected so every persisted value has one predictable shape.
• Trim surrounding whitespace and canonicalize Hex values to lowercase in core. Clients may normalize earlier for immediate feedback, but core remains authoritative.
• Do not expand quick session color tags. Their six-value catalog remains a compact ordering/filter dimension and stays backward compatible.
• Keep the sidecar schema version at 1. The stored field remains a string and older preset values remain valid.
• Existing clients that do not recognize a Hex class should fail safely. The WebShell renders Hex group dots through an inline background-color.

Files

• packages/core/src/services/session-organization-service.ts
• packages/core/src/services/session-organization-service.test.ts
• packages/cli/src/serve/routes/session.ts
• packages/cli/src/serve/acp-http/dispatch.ts
• packages/cli/src/serve/server/session-list.ts
• packages/acp-bridge/src/bridgeTypes.ts
• packages/sdk-typescript/src/daemon/types.ts
• packages/sdk-typescript/src/daemon/index.ts
• packages/sdk-typescript/src/index.ts
• packages/web-shell/client/components/sidebar/WebShellSidebar.tsx
• packages/web-shell/client/components/SessionOverviewPanel.tsx
• packages/web-shell/client/components/sidebar/WebShellSidebar.module.css
• packages/web-shell/client/components/sidebar/WebShellSidebar.test.tsx
• packages/web-shell/client/i18n.tsx

Out of scope

• Custom colors for quick session tags.
• Alpha channels, gradients, named CSS colors, or short Hex forms.
• Changing the group sidecar format or migrating existing values.

Open questions

None. The existing structured error and group persistence paths can be extended without a protocol version bump.