[FEATURE] Persist /color selection across sessions

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The /color command sets a session color, but the preference is never persisted to disk. Every new Claude Code session starts with the default color, requiring the user to manually re-set it each time.

The only workaround for automating this on startup is passing it as a CLI argument (claude /color red), which triggers a full API round-trip just to apply a local UI preference — adding noticeable latency before the session is usable.

Proposed Solution

When /color is invoked, write the selected color to ~/.claude.json and read it back on startup. This is the same file already used as the preferences store for other session settings.

Alternative Solutions

Currently using claude /color red as a CLI argument, but this incurs an unnecessary API round-trip. Third-party tool tweakcc can patch the binary, but breaks on every Claude Code update.

Priority

Low - Nice to have

Feature Category

Configuration and settings

Use Case Example

1. I prefer a specific session color for visual clarity
2. I open a new Claude Code session
3. Without this feature, the color resets to default — I must run /color red manually or pass it as a slow CLI argument
4. With this feature, the color persists automatically with no overhead

Additional Context

Related: #1302 (broader theme customization — this is a much narrower ask targeting only the persistence of the existing /color command)