refactor(code-queue): drop implicit-fallback semantics for minimax m2.7

M2.7 and M3 are two parallel configs, not a primary/fallback pair.
Per user feedback, drop the language and any code that framed M2.7 as
a rollback target for M3.

- code-queue-supervision.md, microservices.md, windows-passthrough.md:
  remove 'default/fallback' labels; spell out that M2.7 and M3 are
  parallel, no auto-fallback, switching provider model is an explicit
  ops action.
- scripts/src/code-queue.ts: modelTiers entries for M3 and M2.7 share
  the same 'simple-low-risk' risk; rename
  minimaxM2FallbackCandidate -> minimaxM27Candidate so the dry-run
  command string no longer implies fallback semantics.
- scripts/code-queue-submit-routing-contract-test.ts: drop 'fallback'
  wording in assertions and the 'model registry fallback' check label.

Refs: #189

scripts/src/code-queue.ts

@@ -1584,7 +1584,7 @@ function submitPolicyContract(): SubmitRoutingRecommendation["policyContract"] {
       {
         model: minimaxSubmitModel,
         runner: "opencode",
-        taskRisk: "simple-low-risk-fallback",
+        taskRisk: "simple-low-risk",
         requiredGuards: ["issue is auxiliary only", "evidence required", "no prod/secrets/DB writes", "diff and test review"],
       },
     ],
@@ -7971,8 +7971,8 @@ function codexSubmitTask(args: string[]): unknown {
       },
       commands: {
         submitAsRequested: "remove --dry-run to submit exactly this payload",
-        minimaxCandidate: `bun scripts/cli.ts codex submit --prompt-file <path> --model ${minimaxM3SubmitModel} --dry-run`,
-        minimaxM2FallbackCandidate: `bun scripts/cli.ts codex submit --prompt-file <path> --model ${minimaxSubmitModel} --dry-run`,
+        minimaxM3Candidate: `bun scripts/cli.ts codex submit --prompt-file <path> --model ${minimaxM3SubmitModel} --dry-run`,
+        minimaxM27Candidate: `bun scripts/cli.ts codex submit --prompt-file <path> --model ${minimaxSubmitModel} --dry-run`,
         deepseekCandidate: `bun scripts/cli.ts codex submit --prompt-file <path> --model ${deepseekSubmitModel} --dry-run`,
         gptCandidate: `bun scripts/cli.ts codex submit --prompt-file <path> --model ${gptSubmitModel} --dry-run`,
       },