Feature Request: Plan status indicators (badge/symbol) on session list items

[RockNoggin]

Describe the feature or problem you'd like to solve

When managing multiple sessions, there's no visual way to tell at a glance what stage each session's plan is in. You have to open each session to check progress, which breaks flow when juggling several workstreams.

Proposed solution

Add a small status indicator (symbol with tooltip, or badge) on each session item in the sidebar that reflects the current plan state:

Status	Meaning	Suggested Icon
Planning	Agent is actively generating/refining the plan	🔄 or spinner
PlanReady	Plan has been generated and is awaiting user approval or kickoff	📋 or checkmark-outline
PlanInProgress	Plan has been approved and work is underway	⚙️ or progress indicator
PlanComplete	All plan steps are done	✅ or filled checkmark

Requirements:

• The indicator should be compact enough to not clutter the session list
• A tooltip on hover should show the full status label (e.g., "Plan In Progress")
• Status should update in real-time as the session progresses
• Sessions without a plan (e.g., chat sessions, sessions in interactive/autopilot mode) can omit the indicator or show a neutral state

Example prompts or workflows

1. User has 5 project sessions open
2. Glancing at the sidebar, they can immediately see:
   • Session A: ✅ (done, ready to review/merge)
   • Session B: ⚙️ (agent is executing the plan)
   • Session C: 📋 (plan ready, waiting for approval)
   • Session D: 🔄 (still planning)
3. User clicks Session C to approve the plan, then moves on — no wasted context-switching

Additional context

Related to #981 (Status Sidebar). This is a lighter-weight version focused specifically on plan lifecycle visibility in the existing session list UI.