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/fetch/command.go

@@ -14,10 +14,10 @@ func NewCommand(opts *cli.Options) *cobra.Command {
 		Use:   "fetch",
 		Short: "Fetch current opportunities",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("fetch.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "fetch")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("fetch.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "fetch")
 		},
 	}
 	cmd.AddCommand(commandOnce(opts))