feat(desktop): add first-class Grok Build ACP runtime support

[ravz]

Summary

Adds first-class Grok Build ACP runtime support to the managed-agent catalog and supervised launch path, pinned to the subscription-backed ACP argv contract grok agent --model <model> stdio (discrete arguments, never shell-interpolated, no API-key/credit fallback — the child inherits only the existing local grok.com session).

Harness (crates/buzz-acp/src/config.rs)

• default_agent_args("grok") = the pinned ["agent", "--model", "grok-4.6", "stdio"] default.
• New effective_agent_args(command, agent_args, model) bridges the managed model into the argv as a discrete argument, fails closed on an empty model, and leaves every other runtime byte-identical to normalize_agent_args. Explicit agent_args win.
• Deterministic tests: pinned default, model substitution, empty-model fail-closed, .exe identity normalization, explicit-args precedence, non-grok behavior unchanged.

Desktop (desktop/src-tauri/src/managed_agents/discovery.rs + runtime_metadata.rs)

• Genuine grok entry in KNOWN_ACP_RUNTIMES: provider locked, model_env_var: None (model travels in argv, not env), required_normalized_fields: ["model"], direct ACP runtime (no adapter), login hint.
• ~/.grok/bin added to binary discovery paths (official Grok Build install layout, matching the existing per-runtime well-known-path probes).
• Desktop default_agent_args mirror + catalog metadata tests.

Invariants preserved: no change to the default runtime preference order (getDefaultPersonaRuntime untouched), no alias, no PATH dependence when the exact official binary is discovered, no shell string, no ambient API key, supervised one-child spawn unchanged (identity, system prompt, policy, queue, locks, timeout, cancellation all remain buzz-acp-owned).

Local verification (Windows host): cargo fmt --check clean; cargo test -p buzz-acp --lib at head: 759 passed / 17 failed, and the failure set is a strict subset of the base failure set (31 failures at base — timing/env-sensitive steer tests on this host; BUZZ_ACP_* ambient env), i.e. zero new failures attributable to this change; all new grok tests pass. Desktop crate full suite not runnable locally: the native build dependency cmake is absent on this host (no cmake anywhere on PATH/MSYS2); CI is authoritative for the desktop suite.

[Chessing234]

auth_probe_args: None on the new entry does not mean "no probe", it means no login step. discover_acp_runtimes_from fills AuthStatus::NotApplicable for any available runtime with no probe args, and the enum documents that as "this runtime does not have a login step (e.g. goose, buzz-agent)". grok is underlying_cli: Some("grok") and the body describes it as subscription-backed, so the badge will tell people grok needs no auth while a logged-out cli fails at the first turn — the same false-positive shape #5460 reports for claude.

separately: the default model is pinned in two places, default_agent_args (grok-4.6 in the argv) and effective_agent_args (model.unwrap_or("grok-4.6")). they will drift.

[ravz]

Addressed both points in f0d73ce.

• Grok now probes with grok models. The current CLI exits 0 in both auth states, so the probe now captures stdout and recognizes its explicit You are not authenticated. result before applying the existing exit-code fallback. This fixes both catalog discovery and readiness checks without changing Codex/Claude behavior; config-error precedence remains intact.
• The Grok default model is now defined once (GROK_DEFAULT_MODEL) and both default/effective argv paths use the same grok_agent_args builder.
• While exercising the real clap boundary, I also fixed the legacy cross-runtime agent_args=["acp"] default so it cannot shadow Grok's managed model. Full explicit Grok argv overrides still win.

Local verification:

• cargo test -p buzz-acp --lib config::tests: 123 passed
• auth classifier source tests: 5 passed (logged in, successful-but-logged-out, generic success, config-error precedence)
• cargo clippy -p buzz-acp --lib -- -D warnings: passed
• root + desktop Rust formatting and diff checks: passed

The full desktop crate still cannot link on this Windows host because the unrelated native sherpa-onnx-c-api static library is unavailable; CI is authoritative for that complete build.