refactor(migration): auto-upgrade claude-opus-4-5 and 4-6 to claude-opus-4-7

MODEL_VERSION_MAP now chains the legacy claude-opus-4-5 entry straight
to claude-opus-4-7 and adds an explicit claude-opus-4-6 to 4-7 bump
path, letting existing user configs upgrade on next load without an
intermediate 4-6 stop.

MODEL_TO_CATEGORY_MAP picks up claude-opus-4-7 as the canonical
unspecified-high model (prior 4-6 entry is covered by the chained
version map above, so legacy hardcoded configs still resolve).

Migration tests rewritten to reflect the chained 4-5 to 4-7 behavior
and the new 4-6 to 4-7 bump path, including the sidecar-union
scenario.

src/shared/migration/agent-category.ts

@@ -16,7 +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",
 }