P4: contrib/store — second module (pure-Go SQLite), budget store

Establish the nested persistence module — the architectural reason the core
stays lean: a SEPARATE go.mod carrying modernc.org/sqlite (pure Go, no cgo), so
the SQLite driver NEVER enters the executus core go.sum. A static-binary host
(gadfly) importing only the core stays static; a host wanting turnkey
persistence imports contrib/store.

- sqlite.go: store.Open(dsn) -> *DB (one SQLite file), accessor-per-seam.
- budget_store.go: db.Budget() satisfies budget.BudgetStorage; Add() does the
  7-day window rollover atomically inside a transaction (concurrent Adds can't
  race the read-modify-write — the in-memory store's one weak spot).
- Conformance test: budget.NewDBBudget over the SQLite store passes the SAME
  rolling-window contract as the in-memory store.
- CI: a new step builds + tests contrib/store on its own AND asserts it carries
  the sqlite driver the core forbids (proof the split works). Verified: core
  go.sum has 0 sqlite refs; contrib/store go.sum has it.

persona/skill/audit SQLite stores follow next (same JSON-blob + indexed-columns
pattern, sidestepping the three-layer field-loss footgun).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -75,8 +75,10 @@ BATTERIES (opt-in siblings, each nil-safe + a default):
   budget/    DBBudget rolling-7d + NoOp (run.Budget);    [P4 ✓]
              BudgetStorage iface + Memory default
 
-contrib/store/   SECOND module (+ modernc.org/sqlite):   [P4]
-  in-memory + pure-Go SQLite impls of every *Store seam
+contrib/store/   SECOND module (+ modernc.org/sqlite):   [P4 ~]
+  pure-Go SQLite impls of the *Store seams. budget ✓
+  (conformance-tested); persona/skill/audit pending.
+  CI proves the driver lands HERE, not in the core go.sum.
 ```
 
 ### The one architectural move