docs(roadmap): add #360 — /tokens, /stats, /cost identical output; no context-window or cost_usd

[code-yeongyu]

Pinpoint #349

Three commands with distinct names produce identical JSON; none includes context-window utilisation.

Live on a510f73:

/tokens → {kind:"stats", input_tokens:0, ...}  // same as /stats
/stats  → {kind:"stats", input_tokens:0, ...}
/cost   → {kind:"cost",  input_tokens:0, ...}  // same payload, different kind
// context_window: absent  context_remaining: absent  cost_usd: absent

Caller cannot check context headroom, cannot budget spend, cannot distinguish turn counts from token costs — all three commands are the same blob.

ROADMAP.md only