P4: budget battery — DBBudget (rolling 7-day) over run.Budget

Second Tier-2 battery, plugging into run.Ports.Budget:
- budget.go: skillexec's BudgetTracker / NoOpBudget / DBBudget moved clean
  (stdlib only). Check/Commit match run.Budget exactly (compile-time proof in
  run.go: NoOpBudget and *DBBudget are run.Budget).
- storage.go: the BudgetStorage seam + SkillBudget domain, split out of mort's
  GORM file (the GORM impl stays in mort).
- memory.go: NewMemory() — zero-dependency in-process BudgetStorage with the
  7-day rolling-window rollover in Add.

Tests: per-user cap enforced, window rolls over after 7 days, NoOp always
allows. CI invariant: core imports ZERO from the budget battery.

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

CLAUDE.md

@@ -75,7 +75,8 @@ BATTERIES (opt-in siblings, each nil-safe + a default):
   critic/    two-tier timeout state machine + Escalator  [P4]
   schedule/  cron runner cores                           [P4]
   checkpoint/ durable resume seam                        [P4]
-  budget/    rolling-window tracker (+ NoOp)             [P4]
+  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