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-shell/src/session/acp_session_impl/turn.rs

@@ -394,15 +394,6 @@ impl SessionActor {
             schema_version: crate::session::events::EVENT_SCHEMA_VERSION.into(),
             redirect_kind,
         });
-        self.observability_bridge
-            .emit(
-                kigi_tool_protocol::session_event::SessionEvent::TurnStarted {
-                    turn_number,
-                    model_id: model_id.clone(),
-                    yolo_mode,
-                },
-            )
-            .await;
         self.send_before_turn_event(kigi_tool_protocol::turn_hook::BeforeTurnPayload {
             turn_number: self.chat_state_handle.get_prompt_index().await as u64,
             model_id: model_id.clone(),
@@ -733,15 +724,6 @@ impl SessionActor {
         );
         let turn_tool_count = self.events.tool_count_this_turn();
         let bridge_outcome = turn_result_to_hook_outcome(&result);
-        self.observability_bridge
-            .emit(kigi_tool_protocol::session_event::SessionEvent::TurnEnded {
-                turn_number: current_prompt_index as u64,
-                outcome: bridge_outcome,
-                duration_ms: turn_duration_ms,
-                tool_call_count: turn_tool_count,
-                model_id: turn_model_id.clone(),
-            })
-            .await;
         match &result {
             Ok(TurnOutcome::Completed { .. }) => {
                 self.emit_turn_ended(
@@ -1678,13 +1660,6 @@ impl SessionActor {
             self.emit_event(crate::session::events::Event::PhaseChanged {
                 phase: crate::session::events::Phase::WaitingForModel,
             });
-            self.observability_bridge
-                .emit(
-                    kigi_tool_protocol::session_event::SessionEvent::PhaseChanged {
-                        phase: kigi_tool_protocol::session_event::SessionPhase::Sampling,
-                    },
-                )
-                .await;
             kigi_log::unified_log::info(
                 "shell.turn.inference_start",
                 Some(self.session_info.id.0.as_ref()),
@@ -1989,13 +1964,6 @@ impl SessionActor {
             self.emit_event(crate::session::events::Event::PhaseChanged {
                 phase: crate::session::events::Phase::ToolExecution,
             });
-            self.observability_bridge
-                .emit(
-                    kigi_tool_protocol::session_event::SessionEvent::PhaseChanged {
-                        phase: kigi_tool_protocol::session_event::SessionPhase::ToolExecution,
-                    },
-                )
-                .await;
             let execute_tool_calls_result = self.execute_tool_calls(tool_call_responses).await;
             match execute_tool_calls_result {
                 Ok(ToolLoop::PermissionReject { tool_name, reason }) => {