review: update default model prices to current Anthropic list pricing

Per operator request on the PR: the built-in/default prices were the old
Claude 3 numbers (opus 15/75, etc.). Update opus + haiku to the current
Claude 4.x family list pricing (cache_write = the default 5-minute cache
TTL); sonnet was already correct:

- opus:   input 5, output 25, cache_read 0.5,  cache_write 6.25
- sonnet: input 3, output 15, cache_read 0.3,  cache_write 3.75  (unchanged)
- haiku:  input 1, output 5,  cache_read 0.1,  cache_write 1.25

Updated in both builtin_prices (hive_stats.rs) and the nix modelPrices
default (hive-c0re.nix), with cross-reference "keep in sync" notes on
both sides. Also addresses the earlier reviewer note: dropped the
over-strong "single source of truth" wording in the --model-prices arg
doc (the nix default does mirror the numbers in production).

hive-c0re/src/main.rs

@@ -89,9 +89,10 @@ enum Cmd {
         /// model id; models not covered fall back to the built-in
         /// estimate. Set via the `services.hyperhive.modelPrices` NixOS
         /// option (whose default carries the full opus/sonnet/haiku
-        /// table). Defaults to `{}` here so a bare `hive-c0re serve`
-        /// leans entirely on the in-code `builtin_prices` fallback —
-        /// keeping a single source of truth for the numbers.
+        /// table — kept in sync with the in-code `builtin_prices`).
+        /// Defaults to `{}` here so a bare `hive-c0re serve` leans
+        /// entirely on `builtin_prices`; the NixOS default (every real
+        /// deployment) shadows it with the same operator-tunable table.
         #[arg(long, default_value = "{}")]
         model_prices: String,
     },