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.

cmd/jobsearch/main.go

@@ -52,17 +52,16 @@ opportunities.
 		PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
 			cli.SetupInitialLogging(cmd)
 
-			slog.Debug("root.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "root")
 			slog.Debug(
-				"command: \"" +
-					strings.Join(strings.Split(cmd.CommandPath(), " ")[1:], " ") +
-					"\"",
+				"command",
+				"path", strings.Join(strings.Split(cmd.CommandPath(), " ")[1:], " "),
 			)
 
 			return nil
 		},
 		PersistentPostRunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("root.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "root")
 
 			return opts.Finish()
 		},