fix: update v2 session usage metrics

[rekram1-node]

Summary

• calculate V2 step costs from catalog pricing and context tiers
• prefer Copilot provider-reported AIU billing when available, with token pricing as fallback
• persist session token/cost totals after each completed step and refresh authoritative totals in the TUI
• prevent stale concurrent session snapshots from overwriting newer usage
• read sidebar and subagent context usage from V2 messages and models
• follow transcript-order undo and redo boundaries for context tokens while preserving cumulative cost

Testing

• core session projector and runner tests
• TUI data race and undo ordering regression tests
• focused package checks and interactive V2 TUI smoke test