improvement(ci): bound the local Turborepo cache (#7214)

Turborepo's local cache eviction is opt-in until 3.0, so without cacheMaxAge and
cacheMaxSize the filesystem cache grows forever. In CI each cache dir lives on a
Blacksmith sticky disk that is mounted many times a day, so it never idles long
enough for Blacksmith's own 7-day inactivity purge to fire, and one cache-missing
app build writes a ~400 MB artifact. The build cache disk reached 206 GB over 43
days — roughly 4.8 GB/day of sediment — at ~$0.51/GB-month.

Size is the real bound; age is hygiene. A cache hit only happens when a task's
input hash is unchanged, which recurs within hours, not weeks, so nothing written
days ago can ever be read again — the sibling PR-keyed disk does the same job in
30 GB. The cache still earns its keep: the app build hits ~17% of the time and a
hit saves ~7 minutes, so this is a ceiling, not a removal. That distinguishes it
from the Turbopack persistent cache, which was removed because it measured 3.2x
SLOWER; this one is measurably faster, just unbounded.

Set in turbo.json rather than per-step env vars so there is one source of truth
and turbo validates it. Verified against the installed 2.9.14: both keys are in
its schema, turbo parses them, and the task hash is byte-identical with and
without them, so enabling eviction does not invalidate the existing cache.

turbo.json

@@ -1,6 +1,23 @@
 {
   "$schema": "https://v2-9-12.turborepo.dev/schema.json",
   "envMode": "loose",
+  // Local cache eviction is opt-in until Turborepo 3.0, so without these the
+  // filesystem cache grows forever. In CI each cache dir is a Blacksmith sticky
+  // disk that is mounted many times a day, so it never goes idle long enough for
+  // Blacksmith's own 7-day inactivity purge to fire, and one cache-missing app
+  // build writes a ~400 MB artifact: the build cache reached 206 GB in 43 days.
+  //
+  // Size is the real bound and age is hygiene. A hit only happens when a task's
+  // input hash is unchanged — a re-run, or a commit touching only unrelated
+  // workspaces — which recurs within hours, so nothing older than a day or two
+  // can ever be read again. Measured hit rate on the app build is ~17%, worth
+  // ~7 minutes each, so the cache earns its keep; it just needs a ceiling.
+  // 40 GB is ~100 app-sized artifacts against ~4.8 GB/day of real accumulation.
+  //
+  // Neither key participates in the task hash, so changing them does not
+  // invalidate the cache.
+  "cacheMaxAge": "7d",
+  "cacheMaxSize": "40GB",
   "tasks": {
     "transit": {
       "dependsOn": ["^transit"],