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)

harnesses/contexts/minimax-m31-flash/v1/harness.yaml

@@ -0,0 +1,17 @@
+kind: context
+name: minimax-m31-flash
+version: 1
+description: "Pin MiniMax-M3.1-Flash-Preview (1M context) — layer AFTER minimax/v1 (auth + base URL come from there). Thinking depth is set per dispatch via --effort (models/*.yaml default_flags.reasoning_effort), not here."
+requires: []
+provides: []
+
+# Replaces minimax-m3-max/v1 (kept for rollback). Differences:
+# - Flash always thinks and ignores MAX_THINKING_TOKENS/budget_tokens (effort is
+#   the only depth control, server default max), so there is no init.sh.
+# - No [1m] suffix: Flash is 1M natively (verified 2026-10-13, modelUsage.contextWindow).
+# - CLAUDE_CODE_AUTO_COMPACT_WINDOW stays: an unknown model id otherwise gets
+#   Claude Code's 200K default and compacts early.
+env:
+  ANTHROPIC_MODEL: "MiniMax-M3.1-Flash-Preview"
+  ANTHROPIC_SMALL_FAST_MODEL: "MiniMax-M3.1-Flash-Preview"
+  CLAUDE_CODE_AUTO_COMPACT_WINDOW: "1000000"