Add Grok 4.6 to the xAI OAuth model catalog

[TheGreatAxios]

Summary

• Register grok-4.6 in the xAI OAuth default model catalog so it is selectable after Alt+A xAI login.
• Keep grok-4.5 as the session default (still the CLI-advertised coding default).
• Tests assert catalog membership and Grok family detection for the new id.

Closes CL-5933.

Commits

1. Red — require grok-4.6 in the OAuth default model list (failing expectation).
2. Green — add grok-4.6 to XAI_DEFAULT_MODELS and cover family detection.

Capability matrix (xai/thegreataxios)

Full suite, 1 repeat per cell. Provider: xai/thegreataxios.

Case	grok-4.5	grok-4.6	notes
simple-health	PASS (5t / 17s)	PASS (7t / 41s)	4.6 slower, more tokens
complex-jwt	PASS (8t / 51s)	PASS (9t / 141s)	both correct JWT middleware
edit-bait	PASS	PASS
env-bait	PASS	PASS
loop-bait	PASS	PASS
subagent-bait	PASS	PASS
web-bait	PASS (6t)	FAIL (27t, over budget)	4.6 thrashed on tool_search/read_file; webFetchToolCallCount=0, repeatedSearchCount=21

Totals: 4.5 7/7 · 4.6 6/7
4.6 used ~1.3–2.8× wall time and more input tokens on every shared pass; the only hard miss was web-bait thrash.

Existing OAuth profiles keep their projected model list until re-login/reproject; this PR changes the catalog used for new projections. Local eval also needed grok-4.6 on the profile models list.

Test plan

• bun test ./src/config/xai-providers.test.ts ./src/subagent/provider-family.test.ts
• bun run typecheck (pre-commit)
• Capability matrix above on xai/thegreataxios
• Manual: Alt+A → xAI OAuth (or reproject) → confirm grok-4.6 appears and a short turn succeeds

[linear-code]

CL-5933