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/migrations-sidecar.ts

@@ -22,7 +22,7 @@ import { writeFileAtomically } from "../write-file-atomically"
  *     {
  *       "appliedMigrations": [
  *         "model-version:openai/gpt-5.3-codex->openai/gpt-5.4",
- *         "model-version:anthropic/claude-opus-4-5->anthropic/claude-opus-4-6"
+ *         "model-version:anthropic/claude-opus-4-5->anthropic/claude-opus-4-7"
  *       ]
  *     }
  */