Configurable colors and thresholds via TOKENLINE_* env vars

[ropdias]

Context

Colors and the gradient thresholds (cache HOT/COLD steps, context 50%/80% bands,
rate-limit 25/50/75/90 bands, saving-% bands) are currently hardcoded. Users with
different terminal themes or risk tolerances can't tune them without editing the
script.

Proposal

• Allow overrides via environment variables read at the top of the script, each
  defaulting to the current hardcoded value, e.g.:
  • TOKENLINE_COLOR_HOT, TOKENLINE_COLOR_COLD, …
  • TOKENLINE_CTX_WARN_PCT (default 50), TOKENLINE_CTX_CRIT_PCT (default 80)
• Keep zero-config behavior identical — env vars are purely additive.
• Document the full list in the README under a "Customization" section.

Acceptance

• With no env vars set, output is byte-identical to today.
• Setting an override changes only the corresponding element.