Attribute nested Cursor and Kimi CLI token usage once either exposes a durable session store

[atomantic]

Problem / Goal

#5831 closed the nested-CLI token-attribution hole for Grok and Antigravity. Cursor and Kimi are still in it: transcriptFamily() returns null for both, so every nested --review-with cursor / kimi pass is recorded in data/usage.json as a chars/4 estimate against the parent provider's bucket, not their own.

#5831 gated them explicitly — "include them in the sibling-family scan only if a durable cwd-keyed session store is found during implementation". While implementing Grok/Antigravity, no such store was found for either CLI, so they were deliberately left out.

Why this is blocked rather than claimable

The work is a mechanical extension of the readers that #5831 landed — but only once there is something on disk to read. Neither CLI is currently known to persist a cwd-keyed, per-session transcript the way ~/.grok/sessions/<urlencoded-cwd>/ or ~/.claude/projects/<cwd-slug>/ do. Until one exists, there is no data source to attribute from, and estimating harder would not improve the numbers.

Acceptance (when unblocked)

• Confirm whether current Cursor / Kimi CLI versions write a durable, cwd-keyed session store containing per-turn token counts (or at least message text).
• If yes: add a reader alongside the Grok/Antigravity ones and extend transcriptFamily() so nested runs are attributed to cursor / kimi in data/usage.json.
• If the store carries cumulative-per-session totals, reuse the delta/high-water handling Attribute nested Grok and Antigravity review-CLI token usage in cost reports #5831 introduced rather than summing snapshots.
• If no store exists: record that finding here and close, rather than leaving this open indefinitely.

Re-check trigger

Revisit on a Cursor or Kimi CLI upgrade, or whenever either is added/promoted as a --review-with reviewer.

Refs #5831