feat: copy session details button in Context Health

[KyleNesium]

Summary

• Copy session details button in Context Health header — clipboard icon next to refresh button copies full session details as aligned plain text
• Output includes exact token counts (not abbreviated), model, project, branch, duration, velocity, warnings, and suggested actions
• Green checkmark feedback for 1.5s after copy
• 6 new tests for SessionInfoFormatter.copyableDetails()

Example clipboard output:

Context Health
─────────────
Session:  abc-12345-def
Model:    Opus 4.6
Project:  AIBattery
Branch:   feat/login
Context:  50000/160000 (31%)
Input:    30000
Output:   20000
Turns:    42
Duration: 2h 15m
Velocity: 350 tok/min

Test plan

• Build and verify copy button appears next to refresh in Context Health
• Click copy → verify clipboard contains formatted session details
• Verify green icon feedback appears for ~1.5s
• Collapsed state: button hidden (only shows when expanded)
• Run swift test — 6 new SessionInfoFormatter tests pass