feat: add graphKey support to tracker events and resumption tokens

[mattrmc1]

Summary

Extends Tracker with an optional graphKey parameter. When non-empty, graphKey is included in every event's trackData and in the resumption token. When empty, it is omitted entirely. This is the foundation for agent-graph node trackers (Task 06), where each node's tracker carries the containing graph's key.

Tracker construction

newTracker and newTrackerWithStopwatch gain a graphKey string parameter (last positional arg). The graphKey is stored on the Tracker struct and conditionally added to trackData:

if graphKey != "" {
    builder.Set("graphKey", ldvalue.String(graphKey))
}

All existing call sites in client.go (returnDefault and evaluateConfig) pass "", so current behavior is unchanged.

Resumption token

resumptionPayload gains a GraphKey field with json:"graphKey,omitempty":

type resumptionPayload struct {
    RunID        string `json:"runId"`
    ConfigKey    string `json:"configKey"`
    VariationKey string `json:"variationKey,omitempty"`
    Version      int    `json:"version"`
    GraphKey     string `json:"graphKey,omitempty"`
}

TrackerFromResumptionToken reads graphKey from the decoded payload and passes it through to the reconstructed tracker.

Test plan

• go test -race ./... passes
• Tracker with graphKey="my-graph" — every emitted event (success, duration, tokens, etc.) includes "graphKey":"my-graph" in data
• Tracker with graphKey="" — graphKey key absent from trackData entirely (not present as empty string)
• Resumption token with graphKey — decoded JSON contains "graphKey":"my-graph"
• Resumption token without graphKey — graphKey key absent from JSON
• TrackerFromResumptionToken with graphKey — reconstructed tracker emits events with graphKey and produces an identical resumption token
• Client.CreateTracker round-trip with graphKey — token survives client-level reconstruction, events include graphKey
• All existing tracker tests pass unchanged (graphKey "" everywhere)

---

Note

Low Risk
Additive optional metadata on existing tracker APIs; default call sites unchanged and covered by tests for token round-trip and event payloads.

Overview
Adds an optional graphKey on internal Tracker construction so agent-graph node runs can tag analytics with the parent graph. When set, graphKey is included in shared trackData for all metric events and in ResumptionToken JSON (omitempty); when empty, the field is omitted entirely.

TrackerFromResumptionToken and Client.CreateTracker round-trips preserve graphKey. Config-driven tracker factories in client.go still pass "", so completion/judge configs behave as before until graph node code supplies a key.

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