minimax: move to MiniMax-M3.1-Flash-Preview; drop bare minimax/minimax-m2 slugs

- new context minimax-m31-flash/v1 (model pin; no thinking-budget init, Flash
  ignores it); four composites switch from minimax-m3-max/v1 (kept for rollback)
- models + registry for minimax-m31-flash-{max,medium}; removes the dangling
  model-registry/minimax.yaml (endpoint "minimax" never existed)
- tiers, harness-roles, minimax-* task templates and the *-review-loop@1
  workflows use the Flash slugs (MiniMax silently maps unknown names to M3, so
  the provider-id translation lands in agent-runtimes)

task-templates/minimax-code-repo.yaml

@@ -1,6 +1,6 @@
 name: minimax-code-repo
 description: "MiniMax code agent — repo clone, agent-repo branch persistence"
-model: minimax
+model: minimax-m31-flash-max
 harness: code-minimax-repo/v1
 requires_tags: [claude-code, coding-agent, git-access, project-repo]
 required_params: [repo_url, agent_repo_url]

task-templates/minimax-code-tdd.yaml

@@ -1,6 +1,6 @@
 name: minimax-code-tdd
 description: "MiniMax code agent with TDD enforcement — tests are read-only and must pass before the task finishes"
-model: minimax
+model: minimax-m31-flash-max
 harness: code-minimax-tdd-repo/v1
 requires_tags: [coding-agent, git-access, project-repo, tdd-gate]
 required_params: [repo_url, agent_repo_url]

task-templates/minimax-planning.yaml

@@ -1,6 +1,6 @@
 name: minimax-planning
 description: "MiniMax planning agent — specs, reviews, architecture docs"
-model: minimax
+model: minimax-m31-flash-max
 harness: planning-minimax-repo/v1
 requires_tags: [claude-code, planning-agent, git-access, project-repo]
 required_params: [repo_url, agent_repo_url]

task-templates/minimax-security-review.yaml

@@ -1,6 +1,6 @@
 name: minimax-security-review
 description: "MiniMax security review agent — security audit via Anthropic-compatible proxy"
-model: minimax
+model: minimax-m31-flash-max
 harness: security-review-minimax-repo/v1
 requires_tags: [claude-code, security-reviewer, best-practices, git-access, project-repo]
 required_params: [repo_url, agent_repo_url]

task-templates/minimax-tdd-impl.yaml

@@ -1,6 +1,6 @@
 name: minimax-tdd-impl
 description: "MiniMax TDD implementation agent — tests locked read-only, cannot be modified by agent"
-model: minimax
+model: minimax-m31-flash-max
 harness: minimax-tdd-impl/v1
 requires_tags: [claude-code, coding-agent, git-access, project-repo]
 required_params: [repo_url, agent_repo_url]

task-templates/minimax-test-writer.yaml

@@ -1,6 +1,6 @@
 name: minimax-test-writer
 description: "MiniMax test writing agent — comprehensive tests with edge cases"
-model: minimax
+model: minimax-m31-flash-max
 harness: test-writing-minimax-repo/v1
 requires_tags: [claude-code, test-writer, best-practices, git-access, project-repo]
 required_params: [repo_url, agent_repo_url]

task-templates/workflow/plan-review-loop@1.yaml

@@ -46,7 +46,7 @@ nodes:
   - id: review-minimax
     kind: agent
     role: plan-review
-    model: minimax-m2
+    model: minimax-m31-flash-max
     runtime_env:
       AGENT_REPO_URL: "git@gitea.oreillyit.nz-ai-enablement:skynet/agent-runtimes-agents.git"
       AGENT_BRANCH: "task/${item.uuid}"

task-templates/workflow/spec-review-loop@1.yaml

@@ -54,7 +54,7 @@ nodes:
   - id: review-minimax
     kind: agent
     role: spec-review
-    model: minimax-m2
+    model: minimax-m31-flash-max
     runtime_env:
       AGENT_REPO_URL: "git@gitea.oreillyit.nz-ai-enablement:skynet/agent-runtimes-agents.git"
       AGENT_BRANCH: "spec/${item.uuid}"

task-templates/workflow/test-write-review-loop@1.yaml

@@ -21,7 +21,7 @@ nodes:
     role: test-write-review  # M36 v0.6 (bug 69370eda fix): role + model replaces
                              # the ungrounded `template: minimax-test-writer`
                              # (a composite that never existed in the catalog).
-    model: minimax-m2
+    model: minimax-m31-flash-max
     runtime_env:
       # WF-COMP-15: stage agents share one per-item work branch so their work
       # accumulates across iterations.