feat(cli): Copilot is readable at session granularity, and reads as not covered

[blafourcade]

As someone measuring a team that uses Copilot
I want its sessions to report their tokens
So that a tool that consumed 21,000 tokens stops appearing beside the ones that consumed nothing

Context

Copilot is declared telemetryLocalRead: { kind: "unsupported" }, on the ground that its file carries outputTokens per turn and nothing per request. That was measured, and it was measured on the wrong file.

A live session on 1.0.80 handed a Stop hook a transcript_path nothing here had read:

~/.copilot/session-state/<session-id>/events.jsonl

What that file holds

Its session.shutdown event carries all four counters:

{"type":"session.shutdown","data":{
  "totalPremiumRequests": 0.33,
  "totalNanoAiu": 2655750000,
  "tokenDetails": {
    "input":       {"tokenCount": 10},
    "cache_read":  {"tokenCount": 0},
    "cache_write": {"tokenCount": 21070},
    "output":      {"tokenCount": 42}
  }
}}

assistant.message carries model and turnId; session.auto_mode_resolved records which model the router chose.

And per-turn figures genuinely do not exist. assistant.turn_end carries {"turnId":"0"} and nothing else, so the original claim was right about what a turn can say — only about the wrong file.

totalPremiumRequests: 0.33 for one request independently confirms the denomination measured across fourteen sessions: premium requests, not currency, and never read as an amount.

Expected

Copilot reports a session's tokens, as one kind: "session" record. The stored shape already has that kind, and already says a "session" line is not a per-request figure.

Actual

not covered, which reads to a user as a tool the framework cannot see at all — while the numbers sit in a file the host hands us the path to.

Acceptance

• A Copilot session yields one kind: "session" record carrying the four counters, from session.shutdown.
• Its capability.local_read says token_counters: true, amount: false, tool_stated_step: false — measured, not assumed.
• totalPremiumRequests is never stored as cost_usd. It is a count times a per-model multiplier, invariant to consumption.
• A report says plainly that Copilot's figure is a session total, not a sum of requests — the contract's two-kinds rule already forbids adding it to request lines, and this is the first tool to produce only the second kind.
• The reader is driven by a captured events.jsonl, redacted like every other fixture here: that file holds the system prompt and the assistant's own messages, and none of it may enter a fixture.

Out of scope

Per-step and per-request attribution for Copilot. Neither exists in the file. The journal would give it a step once #681 lands, and the figure would still be a session total.

Relations

Field	Value
parent	#631
related	#681, #653