config + store: remove dead knobs and stale schema

Three drift items surfaced in review, each dead on arrival and each
worth trusting a little more at v0.1.0.

config: drop MetricsPollInterval. The field was parsed from TOML
(metrics_poll_interval), stored on DaemonConfig, and ignored by every
consumer — only StatsPollInterval drives the background poll loop.
Users setting it in config.toml saw zero effect. Removed from the TOML
surface, the model constant, and the config test.

daemon: delete ensureDefaultImage. No callers, body was `_ = ctx;
return nil`. Dead since whatever flow used to call it got removed.

store: drop packages_path from the images table. The column was
carried by the baseline migration but never referenced by UpsertImage
(no INSERT / UPDATE mention) or any Go model field — a ghost from a
build pipeline that no longer exists. Added migration id=2
(drop_dead_image_columns) with an idempotent dropColumnIfExists
helper: fresh installs run baseline (creates the column) + 2 (drops
it); legacy DBs where the column was never added get a no-op. Updated
the direct-INSERT SQL in TestGetImageRejectsMalformedTimestamp to
drop the column reference, and added a migration test covering both
install paths (fresh + legacy).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/store/migrations.go

@@ -24,6 +24,7 @@ type migration struct {
 // entries — installed DBs key off the id column.
 var migrations = []migration{
 	{id: 1, name: "baseline", up: migrateBaseline},
+	{id: 2, name: "drop_dead_image_columns", up: migrateDropDeadImageColumns},
 }
 
 // runMigrations ensures schema_migrations exists, then applies every
@@ -163,6 +164,55 @@ func migrateBaseline(tx *sql.Tx) error {
 	return nil
 }
 
+// migrateDropDeadImageColumns removes image-table columns that the
+// store never reads or writes. `packages_path` was introduced for a
+// build pipeline that no longer exists; the baseline migration still
+// creates it for historical fidelity, and this migration drops it on
+// new installs + any upgrader that still carries it. Idempotent via
+// dropColumnIfExists so running the migration twice (or against a
+// DB where the column was already gone) is a no-op.
+func migrateDropDeadImageColumns(tx *sql.Tx) error {
+	return dropColumnIfExists(tx, "images", "packages_path")
+}
+
+// dropColumnIfExists is SQLite's "ALTER TABLE DROP COLUMN IF EXISTS"
+// (which the dialect lacks) as a library function. modernc.org/sqlite
+// bundles SQLite 3.42+, which supports plain DROP COLUMN — we add the
+// existence guard so the statement is idempotent across repeat runs
+// and legacy DBs that never had the column in the first place.
+func dropColumnIfExists(tx *sql.Tx, table, column string) error {
+	rows, err := tx.Query(fmt.Sprintf("PRAGMA table_info(%s)", table))
+	if err != nil {
+		return err
+	}
+	defer rows.Close()
+	var found bool
+	for rows.Next() {
+		var (
+			cid       int
+			name      string
+			valueType string
+			notNull   int
+			defaultV  sql.NullString
+			pk        int
+		)
+		if err := rows.Scan(&cid, &name, &valueType, &notNull, &defaultV, &pk); err != nil {
+			return err
+		}
+		if name == column {
+			found = true
+		}
+	}
+	if err := rows.Err(); err != nil {
+		return err
+	}
+	if !found {
+		return nil
+	}
+	_, err = tx.Exec(fmt.Sprintf("ALTER TABLE %s DROP COLUMN %s", table, column))
+	return err
+}
+
 // addColumnIfMissing is SQLite's "ALTER TABLE ADD COLUMN IF NOT EXISTS"
 // (which the dialect lacks) as a library function. Used inside
 // migrations when a column needs to survive a database that went