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-shell/src/auth/device.rs

@@ -29,12 +29,14 @@ pub(crate) fn ascii_header_value(value: &str) -> String {
     }
 }
 
-/// The three device-identity headers sent on every OAuth call.
+/// The three device-identity headers sent on every OAuth call and, via
+/// `agent::config::inject_url_derived_headers`, on every first-party
+/// inference request (mirroring kimi-cli src/kimi_cli/llm.py:317-323).
 ///
 /// Errors when the persistent device id cannot be created (e.g. read-only
 /// `~/.kigi`): the OAuth endpoints require `X-Msh-Device-Id`, so login cannot
-/// proceed without it.
-pub(crate) fn device_headers() -> anyhow::Result<[(&'static str, String); 3]> {
+/// proceed without it. Inference callers treat the error as skip-with-warning.
+pub fn device_headers() -> anyhow::Result<[(&'static str, String); 3]> {
     Ok([
         ("X-Msh-Device-Name", ascii_header_value(&device_name())),
         ("X-Msh-Device-Model", ascii_header_value(device_model())),