feat: default to average downsampling

Average consolidation makes every plotted point the true arithmetic mean of
its interval, which is what the Ganglia-style plot appearance depends on.
LTTB deliberately keeps extremes instead, so a plot downsampled with it
cannot be read as interval means.

resolveResampleAlgo used to return an empty string when neither the user nor
the config had chosen an algorithm, and cc-lib's GetResampler maps that to
LTTB. Route every fallback through config.ResampleAlgo() and ship "average"
as the generated and example default.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/config/schema.go

@@ -98,7 +98,7 @@ var configSchema = `
           "enum": ["low", "medium", "high"]
         },
         "default-algo": {
-          "description": "Default resample algorithm when no user preference is set.",
+          "description": "Default resample algorithm when no user preference is set. Default and recommended: 'average' (RRDTool-style interval averaging).",
           "type": "string",
           "enum": ["lttb", "average", "simple"]
         }