§9 acceptance: grep-zero sweep — every internal x.ai/grok identifier renamed

The PRD's first acceptance gate now holds: grep -RinE '\bx\.ai\b|grok'
crates/ --include='*.rs' → 0 matches (exempt: NOTICE and third-party
license archives, README provenance, and the required 'Based on Grok
Build Open Source' attribution, now sourced from version_attribution.txt).

Wire-visible renames (both sides in this repo, changed in lockstep):
- Auth method id 'grok.com' → 'kimi-code' (AuthMethodKind::KimiCode).
- Every x.ai/* and _x.ai/* ACP ext method and meta key → kigi/* /
  _kigi/* (~200 names; grokShell → kigiShell). Session-file replay keeps
  a read-side alias for the legacy '_x.ai/session/update' method so
  existing updates.jsonl histories load; writes emit only the new name
  (both directions test-pinned).
- Agent types grok-build* → kigi* with a documented legacy-prefix alias
  at resolution time so persisted sessions keep resolving.
- ToolNamespace/BuiltinAgentName GrokBuild* → Kigi* (wire snake_case
  kigi/kigi_concise/kigi_hashline; schema regenerated); grok_build
  implementation dirs renamed to kigi*.
- x-grok-* headers → x-kigi-*, __GROK_* sentinels → __KIGI_*, themes
  grokday/groknight → kigiday/kiginight (old persisted values fall back
  to the default theme), web_fetch allowlist xAI hosts → kimi.com +
  moonshot platforms, changelog CDN → this repo, grok-build changelog
  archives deleted.
- BYOK default endpoint removed: [endpoints] api_base_url is now truly
  optional with NO default — consumers fail fast with the flag name when
  unset (no silent x.ai egress). Mock harnesses inject it explicitly.
- System-prompt identity fixed: 'released by xAI' → 'an unofficial
  community CLI for Kimi' (template + regenerated encrypted form).

Also repaired pre-existing grok-era test debt found by the sweep: the
stale trace_classify default-model pin, the grok-pager UA label test,
pty-harness stale-binary reuse and non-hermetic moonshot routing (a PTY
test could previously reach the real api.moonshot.cn), and the outdated
oauth fixture scope key.

Gates: §9 grep 0; fmt clean; workspace check/clippy 0/0 (-D warnings);
FULL cargo test --workspace: 234 suites, 21,961 passed, 0 failed;
deny advisories ok.

crates/codegen/kigi-shell/src/extensions/billing.rs

@@ -1,4 +1,4 @@
-//! `x.ai/billing` extension handler — Kimi Code usage/quota.
+//! `kigi/billing` extension handler — Kimi Code usage/quota.
 //!
 //! Port of kimi-cli's `/usage` command (kimi-cli `src/kimi_cli/ui/shell/usage.py`):
 //! `GET {coding_api_base_url}/usages` with the OAuth Bearer token, parsed into
@@ -17,7 +17,7 @@ use crate::agent::MvpAgent;
 /// human-readable reset hint (e.g. "resets in 2h 5m").
 ///
 /// `Deserialize` because the TUI parses this back out of the
-/// `x.ai/billing` ext response.
+/// `kigi/billing` ext response.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct UsageRow {
@@ -28,7 +28,7 @@ pub struct UsageRow {
     pub reset_hint: Option<String>,
 }
 
-/// Response for `x.ai/billing`: the parsed usage rows, in display order
+/// Response for `kigi/billing`: the parsed usage rows, in display order
 /// (summary row first when the payload carries one).
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
@@ -55,7 +55,7 @@ pub enum UsageError {
 #[tracing::instrument(skip_all, fields(method = %args.method))]
 pub async fn handle(agent: &MvpAgent, args: &acp::ExtRequest) -> ExtResult {
     match args.method.as_ref() {
-        "x.ai/billing" => {
+        "kigi/billing" => {
             tracing::info!("handling usage request");
             handle_get_usage(agent).await
         }