Add cost-proxy token guidance

import (
    "context"
    "encoding/json"
    "errors"
    "fmt"
    "io"
    "strconv"
)

func newCheckpointTokensCmd() *cobra.Command {
    var jsonFlag bool
    var compareFlag string
    var agentBriefFlag bool

cmd := &cobra.Command{
        Use:   "tokens <checkpoint-id>",
        ...
    }

cmd.Flags().BoolVar(&jsonFlag, "json", false, "Output as JSON")
    cmd.Flags().StringVar(&compareFlag, "compare", "", "Compare against a baseline checkpoint ID")
    cmd.Flags().BoolVar(&agentBriefFlag, "agent-brief", false, "Output compact next-step guidance for agents")
    return cmd
}

func runCheckpointTokens(ctx context.Context, cmd *cobra.Command, checkpointIDPrefix string, jsonOutput bool, comparePrefix string) error {
    ...
}

func writeCheckpointTokensAgentBrief(w io.Writer, report checkpointTokensReport) {
    fmt.Fprintln(w, "Checkpoint token brief")
    ...
}

func buildCheckpointMetricDelta(baseline, current int) *checkpointTokensMetricDelta {
    delta := &checkpointTokensMetricDelta{
        Baseline:  baseline,
        Current:   current,
        ...
    }
    return delta
}

func TestRecommendationRules_CacheWritePressure(t *testing.T) {
    t.Parallel()

recs := recommendationRules(tokenRecommendationSignals{
        Tokens: &sessionTokensUsage{
            Total:      50_000,
            CacheWrite: 6_000,
        },
    })

if !recommendationsIncludeID(recs, "cache-write-pressure") {
        ...
    }
}

func TestTokensCmd_AgentBriefPrioritizesNextAction(t *testing.T) {
    ...
}