fix: add persistent cost to prevent under-reporting spent value

[IdrisGit]

What does this PR do?

This PR adds a persistent cost field to sessions to fix incorrect cost reporting in the sidebar when sessions exceed 100 messages. Previously, cost was calculated from the last 100 messages only, causing under-reporting for long sessions.

Key changes:

• Added cost field to Session schema
• Sessions now accumulate cost incrementally when assistant messages complete
• Sidebar reads cost directly from session metadata instead of summing messages
• Forked sessions inherit parent's accumulated cost
• Added startup migration to backfill existing sessions

How did you verify your code works?

Verified with before/after testing on the same long session:

• v1.1.12: Sidebar showed $0.12 (incorrect - last 100 messages only)
• Local build: Sidebar shows $1.61 (correct - accumulated total cost)
• Used local build to work on an issue in the same session, cost is being accumulated correctly.

OpenCode v1.1.12

OpenCode local

Fixes #7767 #6989