Loki Mode

Live in-terminal build HUD

loki start ./prd.md now shows a live per-iteration status line in an interactive
terminal, so the build is no longer silent between the start banner and the end
summary. Each iteration prints one clean line:

[HUD] REASON | iter 3/1000 | $0.42 | +180/-24 (4 files) | took 37s | elapsed 2m11s

• Fields: RARV phase, iteration N/max, cumulative cost (the same total the
  dashboard shows), files changed (+/-) since the run started, this iteration's
  duration, and total elapsed. ETA is shown only when an explicit small
  LOKI_MAX_ITERATIONS is set. Any field whose data is unavailable (non-Claude
  provider with no cost, non-git dir) is omitted rather than faked.
• TTY-gated and opt-out: the HUD renders only on an interactive foreground TTY
  and never in CI, pipes, or --bg. Off-TTY output is byte-identical to before
  (verified). Set LOKI_HUD=0 to disable.
• It is an append-only line at each iteration boundary (not a repainting status
  bar), so it does not fight the agent's streamed output, and it is never written
  to the logs or dashboard. Reviewed by a 3-reviewer council to unanimous approve.