Implement ZCode as a supported provider

[doublezz10]

An ACP layer to interact with ZCode was recently released (https://github.com/william0wang/zcode-acp), wondering if there is interest to add this to T3 Code for native GLM support and a highly tuned agent layer to take best advantage of it.

[doublezz10]

I put together a concrete version of this to make the cost easy to judge: #17377 (draft, not asking for a merge without approval).

It's meant to answer the reasons #8116 and #11272 were declined:

• No new protocol. It's a small flavor over the existing AcpAdapterV2, like Grok and Antigravity, and launches the community zcode-acp-server bridge, which drives ZCode's own app-server.
• No auth in T3. The bridge uses the credentials the ZCode desktop app manages.
• No packaging. Users install the bridge themselves; updates are manual-only.
• One web client definition, no mobile changes. About 1.3k lines including tests and docs.

What it adds over running the bridge as a local ACP command: real permission modes (ZCode's build/edit/yolo; T3's Auto mode is hidden because ZCode's auto denies every tool), no session-opening status probes, models discovered from live sessions, the bridge's per-turn status line dropped, and GLM usage caps reported as Limited. I verified it live on a GLM Coding Plan account; details are in the PR.

If you'd rather ZCode only be reachable through the ACP Registry, that works too: the bridge author already has registry assets, and I'll close the PR.