M2 audit: excise the Computer Hub stack — Kigi's last remote-cloud surface

Removed root-and-branch for the zero-egress guarantee (the hub was xAI's
remote-workspace/cloud-sandbox service):

- Crates deleted: kigi-computer-hub-core, kigi-computer-hub-sdk,
  kigi-computer-hub-mcp-adapter, kigi-workspace-client (hub-proxied
  workspace RPC client), and kigi-tracing (its sole network path was the
  OTLP gRPC exporter; zero consumers remained). kigi-tracing-macros
  (purely local) stays.
- kigi-workspace: every hub surface deleted — hub server/channel/auth,
  HITL-over-hub permissions, donation/metrics pumps, file upload RPCs,
  hub tool-snapshot merge (resolve pipeline is MCP-only now),
  WorkspaceOps::Proxy. Local worktrees, sessions, leader IPC, MCP, and
  the ACP permission prompt path are untouched; LocalRegistry re-homed
  into kigi-tool-runtime on the existing ToolDyn types so in-process
  tool dispatch is unchanged.
- kigi-shell: leader workspace-exposure control surface (incl. the
  wss://computer-hub... URL), [hub] config, ObservabilityBridge, hub
  WebSocket proxy, dead OTLP config knobs. ClientMode::Headless (never
  constructed) removed.
- kigi-tui/bin: hidden `kigi workspace` command removed (`kigi
  worktree` stays).
- Renames: --xai-api-base-url → --api-base-url / KIGI_API_BASE_URL /
  [endpoints] api_base_url (serde alias keeps old configs working; the
  flag feeds BYOK/custom-endpoint routing, not main inference);
  grok_version → kigi_version in inspect/models-cache/trace metadata
  (old caches self-heal via version-mismatch refetch).
- Dependency tree: dropped fastrace*, opentelemetry-otlp/http/proto,
  tokio-tungstenite from the workspace; fixed the 4 real useless_format
  violations the fastrace lint allowance was masking and removed the
  allowance.
- marketplaceAllowlist kept: it gates the LOCAL plugin-marketplace
  feature, not an xAI service.

Known §9 leftover (deliberate, for the M3 sweep): the BYOK default base
URL string. Gates: workspace check/clippy 0/0, fmt, deny ok; suites
green (workspace 1042, shell 4918, tui 6634, tools 2608, tool-runtime
47, mcp 154).

crates/codegen/kigi-pager-pty-harness/src/content.rs

@@ -91,7 +91,7 @@ impl ContentController {
             // KIGI_SHARE_DIR is set in the test runner's env).
             ("KIGI_SHARE_DIR".into(), kigi_home),
             ("KIGI_CODE_BASE_URL".into(), self.url()),
-            ("KIGI_XAI_API_BASE_URL".into(), self.url()),
+            ("KIGI_API_BASE_URL".into(), self.url()),
             ("XAI_API_KEY".into(), "test-key-for-ci".into()),
             ("KIGI_TELEMETRY_ENABLED".into(), "false".into()),
             ("KIGI_FEEDBACK_ENABLED".into(), "false".into()),
@@ -279,7 +279,7 @@ mod tests {
             content.home().join(".kigi").to_str()
         );
         assert_eq!(get("KIGI_CODE_BASE_URL"), Some(content.url()));
-        assert_eq!(get("KIGI_XAI_API_BASE_URL"), Some(content.url()));
+        assert_eq!(get("KIGI_API_BASE_URL"), Some(content.url()));
         assert_eq!(get("XAI_API_KEY").as_deref(), Some("test-key-for-ci"));
         assert_eq!(get("KIGI_TELEMETRY_ENABLED").as_deref(), Some("false"));
         assert_eq!(get("KIGI_FEEDBACK_ENABLED").as_deref(), Some("false"));