Per-directory/project default for /color

\/color\ sets the session's UI accent color, which also drives the row highlight in FleetView's job list — useful for visually grouping sessions by project at a glance. Right now it's session-scoped only: every new session/job starts uncolored and has to run \/color <name>\ manually to get grouped correctly.

Requesting a way to set a default session color per directory/project (e.g. a \color\ field in \.claude/settings.json\, similar to how \model\ already works), so new sessions/jobs in that directory inherit the color automatically instead of requiring a manual \/color\ call every time.