docs(rust/README): document xAI and Kimi model aliases

[nankingjing]

What

The Model Aliases table in rust/README.md only listed the three Anthropic
aliases (opus, sonnet, haiku), but resolve_model_alias in
rust/crates/api/src/providers/mod.rs also resolves xAI and Kimi aliases. This
PR documents the full alias set, adds a Provider and Auth env var column
so each alias is actionable, and notes that resolution is case-insensitive.

Newly documented aliases (verified against MODEL_REGISTRY / resolve_model_alias):

Alias	Resolves To	Provider	Auth env var
grok, grok-3	grok-3	xAI	XAI_API_KEY
grok-mini, grok-3-mini	grok-3-mini	xAI	XAI_API_KEY
grok-2	grok-2	xAI	XAI_API_KEY
kimi	kimi-k2.5	DashScope (OpenAI-compatible)	DASHSCOPE_API_KEY

Why it's correct

Everything added is verifiable in the source, not invented:

• resolve_model_alias() maps grok/grok-3 → grok-3, grok-mini/grok-3-mini
  → grok-3-mini, grok-2 → grok-2, and kimi → kimi-k2.5.
• MODEL_REGISTRY sets the auth/base-url env vars for each provider:
  xAI → XAI_API_KEY / XAI_BASE_URL, Kimi → DASHSCOPE_API_KEY /
  DASHSCOPE_BASE_URL.
• Case-insensitivity: resolve_model_alias lowercases input before matching, and
  existing tests assert this (kimi_alias_resolves_to_kimi_k2_5 checks "KIMI",
  resolves_grok_aliases checks the grok family).
• Provider diagnostics already point users at these aliases (e.g. the hint to
  "use an xAI model alias such as --model grok"), and docs/MODEL_COMPATIBILITY.md
  documents the corresponding request handling — this PR just surfaces the aliases
  where they are listed.

Scope

Docs-only change to rust/README.md. No code or behavior changes. The existing
opus/sonnet/haiku rows are unchanged.

[1716775457damn]

Good catch — the model aliases table was indeed incomplete. Having the full alias set documented alongside the Provider and Auth env var columns makes it much more actionable for users. The case-insensitivity note is also a helpful detail.

[simawulei]

我已收到您的邮件~会尽快给您回复。祝您六时吉祥，阿弥陀佛。

[nankingjing]

Thanks for the quick review, @1716775457damn! Glad the Provider / Auth-env columns are useful. I noticed a couple of other small doc nits while in there (a broken link and an out-of-date crate count) and have a follow-up PR incoming, kept separate to keep this one focused.

[1716775457damn]

Sounds good, keeping the follow-up PR focused on those doc nits separately makes sense. Looking forward to reviewing it — especially the broken link fix. Thanks for the thorough docs work!

[nankingjing]

Quick note: #3269 is just the crate-count fix; I haven't opened the broken-link follow-up yet — will get to that separately. Thanks for flagging it!