fix(core): heal missing pricing across agents

[xingkaixin]

Summary

• add a shared pricing-capture epoch so every agent re-heals legacy cached heads once
• capture missing model pricing per Cursor, OpenCode, and ZCode session
• invalidate cached heads when a previously missing model becomes billable

Root cause

Pricing-miss metadata was only captured by the filesystem synchronization path. Legacy file-agent caches had no marker proving they used that capture behavior, while database agents stored only a session ID and database path, leaving zero-cost heads permanently cached after pricing became available.

Verification

• pnpm lint
• pnpm format:check
• pnpm test
• pnpm build

Issue: CS-185