Per-session cost readout, behind a flag, for API billing #121

The footer shows the model and how much of the context window the last turn left
occupied (ContextRing.tsx, SessionModel.tsx), fed by the session-usage
event (internal/terminal/usage.go). What it never shows is what the session
cost.

Worth stating who this is for before any of it is built: on a subscription the
number is noise. It only means anything on API billing. So it ships off,
behind a setting, and when the flag is off the readout is absent — not zeroed,
not greyed. Nobody on a plan should have to look at a running total that does
not apply to them.

What exists

internal/terminal/usage_claude.go:124 already tails Claude's transcript and
reads input_tokens, cache_read_input_tokens and
cache_creation_input_tokens off the last assistant line. Three things are
missing for cost:

• output_tokens is not parsed at all.
• The reader answers how full is the context right now, so it reads the tail.
  Cost is a sum across every assistant line of the transcript — a different
  traversal, and it has to keep meaning something when /clear starts a new
  transcript mid-session.
• A price table per model. That is a maintenance tail with a known failure: a
  model lich has never heard of prices at nothing.

Open

• Per session only, or a project / daily total as well?
• Price table baked into the binary (goes stale between releases) or a setting?
• Claude only at first — it is the only provider reporting a session id today
  (see the emitUsage doc) — or hold until a second reader exists?
• Where it renders: beside the context ring in the footer, or on the card?