§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test could previously reach the real api.moonshot.cn), and the outdated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-agent/src/prompt/user_message.rs

@@ -5,7 +5,7 @@
 //! workspace overview, optional rules / skills / MCP listings).
 //!
 //! `UserMessageTemplate` selects the rendering strategy:
-//! - `Default` -- the legacy Grok Build prefix (built by the shell layer).
+//! - `Default` -- the legacy Kigi prefix (built by the shell layer).
 //! - `Custom`  -- caller-supplied template string (MiniJinja, same delimiters
 //!   as the system prompt templates).
 //!
@@ -63,7 +63,7 @@ fn normalize_git_status(status: &str) -> Option<String> {
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Default)]
 #[serde(rename_all = "snake_case")]
 pub enum UserMessageTemplate {
-    /// Legacy Grok Build prefix: `<user_info>` + optional `<git_status>`.
+    /// Legacy Kigi prefix: `<user_info>` + optional `<git_status>`.
     /// Built directly by the shell layer; this
     ///   renderer returns `None` for `Default` and the caller falls back to
     ///   its own legacy path.