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/docs/settings.md

@@ -31,6 +31,7 @@ Use `/trust` in interactive mode to save a project trust decision for future ses
 | `defaultModel` | string | - | Default model ID |
 | `defaultThinkingLevel` | string | - | `"off"`, `"minimal"`, `"low"`, `"medium"`, `"high"`, `"xhigh"` |
 | `hideThinkingBlock` | boolean | `false` | Hide thinking blocks in output |
+| `showCacheMissNotices` | boolean | `false` | Show transcript notices for significant prompt-cache misses |
 | `thinkingBudgets` | object | - | Custom token budgets per thinking level |
 
 #### thinkingBudgets