M1/F1: Kimi Code OAuth device-code flow

Replace the xAI OAuth stack with the Kimi device authorization grant:
- kimi_oauth.rs wire layer (device_authorization + token poll + refresh
  against kigi_env::oauth_host(); client_id per PRD; retryable statuses
  429/5xx with backoff; expired_token restarts authorization)
- X-Msh-Device-{Name,Model,Id} headers; device_id minted uuid4-hex at
  ~/.kigi/device_id (0600)
- Storage: system keyring service `kigi`, entry `oauth/kimi-code`
  (macOS/Windows native backends), atomic-file fallback under ~/.kigi;
  official client's keyring/~/.kimi never touched
- Refresh manager: 60s tick, threshold max(300, expires_in*0.5),
  401-tombstone keyed by rejected refresh token with 300s cooldown and
  rotation auto-clear, cross-process lock with sibling-adoption
  triple-check, sleep/wake forced refresh
- Deleted xAI machinery: enterprise OIDC (PKCE/JWKS/teams), devbox login,
  external auth provider, JWT tier gating + subscription paywall stack,
  X-XAI-Token-Auth marker headers, ZDR gates, /user enrichment
- kigi login / TUI /login both drive the device flow; login-host display
  now derives from kigi_env::oauth_host()
- 264 auth unit/wiremock tests; live contract probe of
  auth.kimi.com/api/oauth/device_authorization matches the wire shapes

Gates: check/clippy --all-targets clean, fmt, deny ok, kigi-shell lib
5131 tests green.

Cargo.lock

@@ -1960,6 +1960,16 @@ dependencies = [
  "unicode-segmentation",
 ]
 
+[[package]]
+name = "core-foundation"
+version = "0.9.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91e195e091a93c46f7102ec7818a2aa394e1e1771c3ab4825963fa03e45afb8f"
+dependencies = [
+ "core-foundation-sys",
+ "libc",
+]
+
 [[package]]
 name = "core-foundation"
 version = "0.10.1"
@@ -5509,6 +5519,20 @@ dependencies = [
  "thiserror 2.0.18",
 ]
 
+[[package]]
+name = "keyring"
+version = "3.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eebcc3aff044e5944a8fbaf69eb277d11986064cba30c468730e8b9909fb551c"
+dependencies = [
+ "byteorder",
+ "log",
+ "security-framework 2.11.1",
+ "security-framework 3.7.0",
+ "windows-sys 0.60.2",
+ "zeroize",
+]
+
 [[package]]
 name = "kigi-acp-lib"
 version = "0.1.0"
@@ -5757,7 +5781,7 @@ version = "0.1.0"
 dependencies = [
  "base64",
  "blake3",
- "core-foundation",
+ "core-foundation 0.10.1",
  "dunce",
  "kigi-tty-utils",
  "kigi-version",
@@ -6409,6 +6433,7 @@ dependencies = [
  "jsonschema",
  "jsonwebtoken",
  "kanal",
+ "keyring",
  "kigi-acp-lib",
  "kigi-agent",
  "kigi-agent-lifecycle",
@@ -6508,6 +6533,7 @@ dependencies = [
  "walkdir",
  "webbrowser",
  "windows 0.61.3",
+ "wiremock",
  "zstd",
 ]
 
@@ -6814,7 +6840,7 @@ dependencies = [
  "chrono",
  "clap",
  "clap_complete",
- "core-foundation",
+ "core-foundation 0.10.1",
  "criterion",
  "crossterm",
  "derive_more 2.1.1",
@@ -9977,7 +10003,7 @@ dependencies = [
  "openssl-probe",
  "rustls-pki-types",
  "schannel",
- "security-framework",
+ "security-framework 3.7.0",
 ]
 
 [[package]]
@@ -9996,7 +10022,7 @@ version = "0.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "26d1e2536ce4f35f4846aa13bff16bd0ff40157cdb14cc056c7b14ba41233ba0"
 dependencies = [
- "core-foundation",
+ "core-foundation 0.10.1",
  "core-foundation-sys",
  "jni",
  "log",
@@ -10005,7 +10031,7 @@ dependencies = [
  "rustls-native-certs",
  "rustls-platform-verifier-android",
  "rustls-webpki",
- "security-framework",
+ "security-framework 3.7.0",
  "security-framework-sys",
  "webpki-root-certs",
  "windows-sys 0.61.2",
@@ -10178,6 +10204,19 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "security-framework"
+version = "2.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "897b2245f0b511c87893af39b033e5ca9cce68824c4d7e7630b5a1d339658d02"
+dependencies = [
+ "bitflags 2.13.1",
+ "core-foundation 0.9.4",
+ "core-foundation-sys",
+ "libc",
+ "security-framework-sys",
+]
+
 [[package]]
 name = "security-framework"
 version = "3.7.0"
@@ -10185,7 +10224,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b7f4bc775c73d9a02cde8bf7b2ec4c9d12743edf609006c7facc23998404cd1d"
 dependencies = [
  "bitflags 2.13.1",
- "core-foundation",
+ "core-foundation 0.10.1",
  "core-foundation-sys",
  "libc",
  "security-framework-sys",
@@ -12788,7 +12827,7 @@ version = "1.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0fc95580916af1e68ff6a7be07446fc5db73ebf71cf092de939bbf5f7e189f72"
 dependencies = [
- "core-foundation",
+ "core-foundation 0.10.1",
  "jni",
  "log",
  "ndk-context",