[Feature]: Add optional metadata field to SessionUsageData

[nigel-dev]

Problem

Agent plugins that enrich token data from external sources (e.g. Cursor's CSV server export) have no way to signal to the core UI whether a session's data is estimated (local heuristic) or enriched (server-verified). The core renders all sessions identically, so users can't tell which token counts are approximate and which are accurate.

This came up building @tokentop/agent-cursor: sessions appear immediately with estimated tokens (text.length / 4), then get backfilled with accurate data from Cursor's server. During the gap, there's no visual distinction.

Proposed solution

Add an optional metadata field to SessionUsageData:

export interface SessionUsageData {
  // ... existing fields ...

  /**
   * Optional plugin-defined metadata passed through to the core.
   * Plugins can use this to signal data quality, enrichment status,
   * or other plugin-specific information the UI may choose to render.
   *
   * Known keys (convention, not enforced):
   *   - `isEstimated` (boolean) — true when token counts are local estimates
   */
  metadata?: Record<string, unknown>;
}

This is a non-breaking, additive change. Existing plugins that don't set metadata are unaffected. The core decides how (or whether) to render metadata — plugins just expose the data.

Alternatives considered

• Prefix sessionName with a marker (e.g. "≈ Session Name") — hacky, mixes display concerns into data
• Use absence of cacheRead/cacheWrite as implicit signal — fragile, breaks for sessions that genuinely have no cache usage
• Add a dedicated isEstimated boolean — too narrow; metadata bag is more extensible for future plugin needs

Area

Type definitions

Scope

Small (config, tweak, single type)

Additional context

Downstream issues:

• ttop: Propagate metadata through AgentSessionAggregate and render an indicator in SessionsTable (separate issue)
• agent-cursor: Set metadata.isEstimated in enrichWithCsvData() (will be implemented once SDK ships the type)

Implementation is ~3 lines in src/types/agent.ts. No runtime behavior change.

[nigel-dev]

completed in #5