feat: feed the context donut from _x.ai/session/info (control-plane, no window cost)

[funkpopo]

Wire the context donut (and its popover) to the CLI’s control-plane
_x.ai/session/info RPC so we can show an accurate, structured window
breakdown without spending a model turn or inflating the context window.

Today the donut mostly rides turn-end _meta.totalTokens, with
signals.json + a hidden /session-info prompt as backups. That works
for a single “used / window” number, but:

• cold restore and post-/compact still lag or need a prompt scrape on older paths;
• there is no way to show where the window went (system / tools / messages / skills / MCP);
• the hidden /session-info prompt is a real inference turn on CLIs that lack the rail — wasteful and easy to get wrong.

Grok CLI ≥ ~0.2.112 exposes _x.ai/session/info as a meter, not a prompt:
repeated calls do not bill, do not append chat history, and do not grow
context.used (probe-verified). This PR prefers that path whenever the
method exists, and keeps the existing sources as an ordered fallback.

What changed

Host / ACP

• AcpClient.getSessionInfo() → _x.ai/session/info; returns "unsupported"
  on -32601 so older CLIs (incl. the Windows pin) degrade cleanly.
• Pure parsers in acp-dispatch.ts:
  • parseSessionInfoRpcResult (flat or one-level nested result)
  • sessionInfoCacheFresh + SESSION_INFO_TTL_MS (3s) so popover spam
    doesn’t hammer the process
• Per-session latch: lastSessionInfoAt, sessionInfoUnsupported
  (reset on a fresh process)

Refresh policy (priority)

1. Turn meta / auto_compact_completed — still the live “used” path mid-session
2. _x.ai/session/info — structured snapshot (used + window + breakdown)
3. signals.json — cold / disk seed
4. Hidden /session-info prompt — legacy only when the RPC is missing

Called on:

• cold session/load (disk first, then upgrade via session/info)
• donut popover open (refreshContextDetails, TTL-gated)
• post-/compact when the live rail didn’t fire
• compact re-prime backup path (unchanged intent, better first hop)

Webview

• Host message contextUsage gains optional breakdown fields
  (systemPromptTokens, toolDefinitionsTokens, messageTokens,
  freeTokens, categories[], autoCompactThresholdPercent).
• Popover gains an In this window section (System / Tools / Messages /
  Skills / MCP / Free / Auto-compact at).
• Opening the donut posts refreshContextDetails; an open popover
  re-renders when a structured snapshot arrives.
• Session swap clears breakdown state so numbers don’t flash across sessions.

Protocol / remote

• refreshContextDetails added to WebviewMsg + remote policy (view).
• Host↔webview contract stays the single source of truth in protocol.ts.