Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/cli/migrate/migrate.go

@@ -26,13 +26,13 @@ Running without a subcommand is the same as running "migrate up": every pending
 migration is applied.`,
 		Args: cobra.NoArgs,
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "migrate")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "migrate")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("migrate.Run")
+			slog.Debug("command run", "command", "migrate")
 			return runStepUp(cmd, opts, 0, false)
 		},
 	}