F3: Kimi inference pipeline + full grok cloud-surface excision

Sampler / inference (PRD F3):
- kimi_compat.rs: single adaptation point for the Kimi chat/completions
  dialect (thinking-field mapping, model_id stripping, empty-content
  tool-call message fix, stream_options.include_usage), with kimi-cli
  source citations
- Rate-limit handling reworked for Kimi/Moonshot semantics; UA kigi/{version}
- /models replaces the xAI models-v2 endpoint everywhere; idle model
  refresh carries X-Msh-* device headers only (X-XAI-Token-Auth and
  x-grok-client-mode/CLIENT_MODE_HEADER machinery deleted)

Cloud-surface excision (PRD §5, zero-egress):
- remote/ conversations lane, cli-chat-proxy-types crate, prod/ dir,
  share command, credit bar: deleted (single local session lane;
  paginate() replaces merge_and_paginate)
- Subscription/tier gate stack deleted end-to-end: AppView
  gate/tier/team/ZDR fields, app/subscription.rs watch loop,
  dispatch/billing.rs paywall + SuperGrok upsell, free-usage-exhausted
  chain, tier-restricted commands, GateInfo, RemoteSettings gate fields,
  SettingsUpdateNotification gate fields
- /privacy + coding-data-sharing setting deleted (backed by a dead xAI
  RPC; Kigi is zero-egress — nothing to share or retain remotely)

Auth UX correctness (user-reported):
- Device-flow fixtures now mirror the live Kimi payload shape
  (https://www.kimi.com/code/authorize_device?user_code=..., verified
  against auth.kimi.com); the fabricated auth.kimi.com/device?code=...
  URLs are gone
- open_browser_detached is a no-op under cfg(test): unit tests drove
  wiremock fixture URLs into the real browser (root cause of the
  "garbage mock link" ABCD-1234 tabs)
- Welcome/pager-minimal rebrand: Grok Build -> Kigi, grok.com ->
  kimi.com, "Sign in to Grok" -> "Sign in to Kimi"

crates/codegen/kigi-tui/src/headless.rs

@@ -838,7 +838,6 @@ pub async fn run_single_turn(
 ) -> Result<()> {
     // Stamp proxy requests as headless before the agent spawns and issues
     // its first request (auth enrichment, model list, etc.).
-    kigi_shell::http::set_process_client_mode_headless();
 
     let cwd = match options.cwd {
         None => std::env::current_dir()?,
@@ -1318,13 +1317,7 @@ pub async fn run_single_turn(
         }
         Some(Err(err)) => {
             let msg = if i32::from(err.code) == RATE_LIMITED_ERROR_CODE {
-                // The -32003 data is the flattened server message; a
-                // free-usage 429 carries the well-known code inline there.
-                if crate::app::acp_error_is_free_usage_exhausted(&err) {
-                    crate::app::FREE_USAGE_USER_MESSAGE.to_string()
-                } else {
-                    rate_limited_user_message(is_api_key_auth).to_string()
-                }
+                rate_limited_user_message(is_api_key_auth).to_string()
             } else {
                 err.to_string()
             };