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>

README.md

@@ -61,7 +61,7 @@ Two tiers in one module (`go.mod` = majordomo + stdlib only):
   ships a default, so you add only what you use.
 
 Persistence that needs a real database lives in a **separate** nested module
-(`contrib/store`, pure-Go SQLite) so the core never drags in a DB driver — a
+(`contrib/store`, pure-Go SQLite — the `budget` store landed first, conformance-tested) so the core never drags in a DB driver — a
 static-binary host (gadfly) stays static.
 
 ## License