feat(metrics): emit hit/miss counters for the Redis installation-token cache

[JSONbored]

The Redis installation-token cache (src/selfhost/redis-token-cache.ts) that lets warm tokens survive restarts emits no metrics, so operators can't see cold-start re-mint rate (the whole point of #perf). Add a gittensory_redis_token_cache_total{result="hit"|"miss"} counter in get(): hit when a valid {token,expiresAtMs} is returned, miss when raw is null OR the parsed shape is invalid.

Deliverables

• Import incr from ./metrics into src/selfhost/redis-token-cache.ts
• Increment gittensory_redis_token_cache_total{result="hit"} on a valid parse and {result="miss"} on null raw and on invalid shape (both nullish arms)
• Do NOT log the token value in any metric label
• Unit tests: hit, miss-on-null, miss-on-malformed-JSON, miss-on-wrong-types

References

• src/selfhost/redis-token-cache.ts:15
• src/selfhost/redis-token-cache.ts:31
• src/selfhost/metrics.ts:48

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size: S · gittensor:feature

[linear-code]

JSO-2191