feat: aggregate ledgers from more than one machine

[kleyt0n]

Design issue — agree the shape before building.

Problem

Someone with a laptop and a desktop gets two disjoint pictures, and a team lead gets none.

Why it matters

The ledger is the one artifact that is safe to move between machines, because it is path-free by construction: src/repo.rs reduces a working directory to an owner/name slug during the scan and discards the path, branch and session id. That property is what makes this possible without compromising anything.

What to do

• Export and import of usage-ledger.json, plus a machine label on day rows.
• Merge dedups on the existing (requestId, message.id) key.
• File in, file out — no server, no account, no sync. If it cannot be done that way it should not be done: surface has no backend and no telemetry, and this must not become the reason it acquires one.
• LEDGER_VERSION is at 4 and older ledgers are discarded, so there is a precedent for the schema change.
• Confirm the export carries nothing the ledger does not already hold.

Acceptance

Two machines' ledgers merge into one dashboard with per-machine attribution, and the merge is idempotent.