Proposal: show Muse token usage and cost on the Usage page

[only21mil]

Problem

Muse turns never appear on Usage. Every other built-in provider has a
usage reader (Codex, Claude Code, Grok, Pi, and the rest), but muse is not a
usage provider kind, so someone who runs Muse in T3 sees no token counts
or cost for it. This is separate from Usage → Limits subscription windows
(#18274, #17397): this is about the per-day token/cost history the other
readers feed.

Proposal

Add a transcripts usage reader for the Muse driver, in the same shape as
the Pi and Grok readers:

• Read Muse's own session.jsonl logs (one model_completed event per
  model step, carrying the step's token counts), keyed per record so copied
  logs count once.
• Add muse to the usage provider kinds and provider lists on server, web
  and mobile, plus a line in the user usage guide.
• Pricing goes through the existing path only. Standard Muse Wonder has a
  public rate and prices from the existing table. Muse Wonder Full has no
  public per-token rate (it is Muse subscription only), so it would show
  tokens unpriced unless the user sets a price in Model prices. No rate is
  hardcoded.

Scope: one new reader file plus the provider-kind plumbing and tests, about
480 changed lines in a draft I have locally. No new settings, no default
changes, no network calls.

Two things I would call out up front:

• Muse reports cache_write_tokens next to the input counts. I cannot
  confirm from the logs whether writes are already included in
  input_tokens; every step in my logs has zero writes, so token totals
  today are unaffected either way, but the split may need revisiting once
  writes appear.
• The reader follows the account's XDG_DATA_HOME for the log location,
  with the usual default, matching how the other readers locate their logs.

Ask

Is this direction and scope acceptable for a PR? Happy to adjust (for
example tokens-only with no pricing hooks, or a smaller first step) before
opening one.