P4b: skill noun + contrib/store (SQLite for budget/persona/skill/audit)

Merges the skill half of the persona/skill pair plus the second nested module.
(Squashed onto main from phase-4b-skill; the audit/budget/persona batteries it
was stacked on already landed via the P4 merge.)

- skill/: clean-redesign Skill noun + LEAN SkillStore (lifecycle/versions/
  schedule only) + ToRunnable + Memory default.
- contrib/store/: separate go.mod carrying modernc.org/sqlite, so the driver
  never enters the core go.sum. db.Budget()/Personas()/Skills()/Audit() back
  all four store seams (JSON-blob + indexed columns; round-trip tested).
  Includes the verified gadfly #5 fixes (AppendVersion tx+UNIQUE+error,
  Mark*ScheduledRun atomic json_set, busy_timeout, NaN guard).
- CI: builds + tests the nested module and asserts it owns the sqlite driver.

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

contrib/store/sqlite.go

@@ -0,0 +1,54 @@
+// Package store provides durable, pure-Go SQLite implementations of executus's
+// battery store seams (audit, budget, persona, skill). It is a SEPARATE nested
+// module so the SQLite driver (modernc.org/sqlite — pure Go, no cgo) never
+// enters the executus core go.sum: a static-binary host (gadfly) that imports
+// only the core stays static, while a host that wants turnkey persistence
+// imports this module and gets every *Store seam backed by one SQLite file.
+//
+//	db, _ := store.Open("file:executus.db?_pragma=busy_timeout(5000)")
+//	defer db.Close()
+//	budgetStore := db.Budget() // satisfies budget.BudgetStorage
+package store
+
+import (
+	"database/sql"
+	"fmt"
+
+	_ "modernc.org/sqlite" // pure-Go driver, registered as "sqlite"
+)
+
+// DB is a handle to one SQLite database backing the executus store seams. Each
+// accessor (Budget(), …) returns a seam implementation sharing this connection.
+// Safe for concurrent use (SQLite serializes writes; busy_timeout handles
+// contention). Construct with Open; close with Close.
+type DB struct {
+	sql *sql.DB
+}
+
+// Open opens (creating if absent) a SQLite database at dsn and returns a DB. A
+// dsn of ":memory:" yields an ephemeral in-memory database. The caller owns the
+// returned DB and must Close it.
+func Open(dsn string) (*DB, error) {
+	sqldb, err := sql.Open("sqlite", dsn)
+	if err != nil {
+		return nil, fmt.Errorf("store: open %q: %w", dsn, err)
+	}
+	// A contended writer should WAIT for the lock, not fail immediately — set a
+	// busy_timeout so concurrent stores don't see spurious SQLITE_BUSY. (The
+	// doc example advertised this; it's now actually applied for every DSN.)
+	if _, err := sqldb.Exec("PRAGMA busy_timeout=5000"); err != nil {
+		sqldb.Close()
+		return nil, fmt.Errorf("store: set busy_timeout %q: %w", dsn, err)
+	}
+	if err := sqldb.Ping(); err != nil {
+		sqldb.Close()
+		return nil, fmt.Errorf("store: ping %q: %w", dsn, err)
+	}
+	return &DB{sql: sqldb}, nil
+}
+
+// Close closes the underlying database.
+func (d *DB) Close() error { return d.sql.Close() }
+
+// SQL exposes the underlying *sql.DB for hosts that need direct access.
+func (d *DB) SQL() *sql.DB { return d.sql }