feat(coding-agent): add prompt cache miss tracking (#6427)

Detect prompt cache misses per turn by comparing each assistant message's
cache reads against the previous request's prompt tokens (core/cache-stats.ts).
Significant misses emit a warning-colored transcript notice at the turn they
occur, noting idle gaps past the cache TTL and model switches when relevant.

/session gained cache statistics: a compact token/cache breakdown with hit
rate, a $-prefixed cost section with per-model cost breakdown, and the
cumulative cost re-billed due to cache misses.

packages/coding-agent/CHANGELOG.md

@@ -8,6 +8,7 @@
 - Added public SDK exports for CLI-equivalent model and scoped-model resolution ([#6201](https://github.com/earendil-works/pi/issues/6201)).
 - Added extension and RPC `agent_settled` events plus session-level idle waiting for fully settled agent runs ([#6363](https://github.com/earendil-works/pi/issues/6363)).
 - Added extension entry renderers for persisted display-only session entries that are rendered in interactive mode without being sent to the model context.
+- Added a `showCacheMissNotices` setting and `/settings` toggle for significant prompt-cache miss transcript notices.
 
 ### Fixed