refactor(models): preserve legacy claude-opus-4-6 aliases and category mapping

Addresses review feedback on #3486:

1. claude-thinking-legacy-alias now matches both claude-opus-4-6-thinking
   and claude-opus-4-7-thinking, canonicalizing both to claude-opus-4-7.
   The previous diff retargeted the regex to 4-7 only, which dropped
   backward compatibility for users still pinned to the 4-6 thinking
   suffix.

2. MODEL_TO_CATEGORY_MAP keeps the claude-opus-4-6 to unspecified-high
   entry alongside the new 4-7 entry. The map is order-independent from
   MODEL_VERSION_MAP, so preserving the 4-6 key avoids relying on a
   specific migration ordering for legacy agent configs.

3. Fix stale 'Claude Opus 4.6' labels and BDD test comments that the
   sed-based bump missed.

src/shared/migration/agent-category.ts

@@ -16,6 +16,7 @@ export const MODEL_TO_CATEGORY_MAP: Record<string, string> = {
   "google/gemini-3-flash": "writing",
   "openai/gpt-5.4": "ultrabrain",
   "anthropic/claude-haiku-4-5": "quick",
+  "anthropic/claude-opus-4-6": "unspecified-high",
   "anthropic/claude-opus-4-7": "unspecified-high",
   "anthropic/claude-sonnet-4-6": "unspecified-low",
 }