fix(telemetry): gate the prompt cache like the model report, and keep the first 20 miss causes

[JDProfresh]

These are the two small follow-ups you flagged on #607, one commit each: promptCache was stored for any session mode even though the model report from the same payload is capability-gated, and more than 20 miss causes 400'd the whole statusline post instead of truncating the list.

Store the prompt cache only for CLIs whose statusline reports it

POST /api/status-telemetry gated the model report (noteReportedModel) on capabilities.statusLineTelemetry but stored promptCache unconditionally. The prompt_cache object rides the exact same statusline payload the exporter POSTs, so it is the same question: does this CLI's statusline speak for it. I reused statusLineTelemetry (the capability that also decides whether the exporter is injected at all) rather than add a new field, resolved once per request into a local and applied to both the model report and the prompt-cache store. No CLI-id branch. A non-capable session (codex) now stores no prompt cache; the rest of the post (plan usage) still lands.

Keep the first 20 miss causes instead of refusing the post

last_miss_cause.causes was z.array(z.unknown()).max(20), so an over-long array failed validation and lost the entire post: the footer text the route answers with, plan usage, the model report and the cache readout. I replaced the .max(20) with a z.preprocess that slices the array to 20 before the array schema validates, so an over-long array costs one bounded slice rather than per-element validation across every entry, and the element bound (z.unknown()) stays intact on the 20 kept. parsePromptCache already keeps the string ones among those.

Tests

• A codex (no statusline exporter) post with a valid prompt_cache stores nothing on the session and still broadcasts plan usage.
• A 25-cause post is accepted (200, not 400) and keeps the first 20 causes in order.

Removed each guard and watched its test go red, then restored.

Verified

On this branch, off today's master (1.41.1, a8a8bb71):

• The full npm test gate passes: 11,260 passed, 12 skipped, 0 failed. typecheck, lint, format:check, check:frontend-syntax, check:browser-excludes and prettier on the touched test pass.
• On a throwaway instance built from this branch (removed after), one claude and one codex session, the same statusline post to each with a warm prompt_cache carrying 25 miss causes: both answered 200 with the footer text, the claude session stored the cache with the first 20 causes in order, and the codex session stored neither the cache nor a model. On master that post is a 400 for both.