feat: Plexum-minimax-provider v0.1 — MiniMax via Anthropic-compat endpoint

Plexum ProviderPlugin that serves MiniMax models through MiniMax's
Anthropic-compatible HTTP endpoint
(https://api.minimax.io/anthropic, or CN api.minimaxi.com). Inspired
by openclaw's extensions/minimax provider-registration, but rewritten
in Go for Plexum's SDK.

internal/anthropic/ (~210 LOC + 6 tests):
- minimal HTTP+SSE Anthropic Messages client (POST /v1/messages,
  stream:true, parses event:/data: SSE frames)
- handles non-2xx as HTTP error; stream errors land as Event{Type:"error"}
- 1 MiB SSE line cap; per-conn 5min timeout

internal/translate/ (~220 LOC):
- CanonicalToAnthropic: canonical.TurnRequest → MessagesRequest
- blockToAnthropic: TextBlock / ToolUseBlock / ToolResultBlock /
  ThinkingBlock → loose ContentBlock map; preserves signatures + cache
  control
- Translator: per-turn state machine; consumes anthropic.Event stream
  and emits canonical.TurnEvent stream (handles thinking blocks +
  tool_use input_json_delta accumulation + signature_delta capture)

cmd/plexum-minimax-provider-plugin/:
- Plugin manifest declares provider.models = [MiniMax-M2.7,
  MiniMax-M2.7-highspeed]
- Backend fixed to "api" (per scope); region "global"|"cn" + base_url
  override supported via config
- HostConfig from <profile>/plugins/plexum-minimax-provider/config.json
  {api_key, region?, base_url?, max_tokens_default?}

scripts/install.sh: build + manifest emit; operator writes config.json
+ allows plugin + adds an agent + restarts.

End-to-end verified against the real key:
1. plexum say --agent-id mini ... → "Hi, I'm MiniMax!"
2. Multi-turn continuity: agent recalled the prior reply
3. Via gateway socket: {"outcome":"text","text":"\n\npong"}
4. Via Fabric channel (alice posts → plugin inbound → mini agent
   → MiniMax → outbound REST → reply visible in bt2-clean seq=11):
     "Hi there! 👋 Fun fact: Octopuses have three hearts, blue blood,
      and neurons distributed throughout their arms—so their tentacles
      can 'think'"

The MiniMax-M2.7-highspeed variant works the same way but hit a
Code Plan rate-limit ceiling during testing (not a plugin issue).

Deferred:
- OAuth (Code Plan portal) — not in v1 scope per request
- MiniMax Portal provider (separate provider id minimax-portal)
- Image / TTS / video / music providers (separate plugins later)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/install.sh

@@ -0,0 +1,77 @@
+#!/usr/bin/env bash
+# Plexum-minimax-provider installer (Phase v0.1).
+#
+# Builds + installs:
+#   ~/.plexum/plugins/plexum-minimax-provider/plexum-minimax-provider-plugin
+#   ~/.plexum/plugins/plexum-minimax-provider/manifest.json
+#
+# Operator then writes the per-profile config:
+#   ~/.plexum/plugins/plexum-minimax-provider/config.json
+#     {"api_key": "sk-cp-..."}
+#
+# Re-runnable. Profile data + config.json are never touched.
+#
+# Flags:
+#   --profile <p>    Override profile root (default ~/.plexum)
+set -euo pipefail
+
+REPO="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd)"
+PROFILE_DIR="${HOME}/.plexum"
+
+while [[ $# -gt 0 ]]; do
+    case "$1" in
+        --profile) PROFILE_DIR="$2"; shift 2 ;;
+        -h|--help) sed -n '2,/^set -euo/p' "$0" | sed -n '/^#/p' | sed 's/^# \{0,1\}//'; exit 0 ;;
+        *) echo "unknown flag: $1" >&2; exit 2 ;;
+    esac
+done
+
+log() { printf '\033[1;34m[minimax-install]\033[0m %s\n' "$*"; }
+command -v go >/dev/null || { echo "go not found on PATH" >&2; exit 1; }
+
+PLUGIN_DIR="${PROFILE_DIR}/plugins/plexum-minimax-provider"
+mkdir -p "${PLUGIN_DIR}"
+
+cd "${REPO}"
+VERSION="$(git describe --tags --always 2>/dev/null || echo dev)"
+LDFLAGS="-X main.Version=${VERSION}"
+log "building plexum-minimax-provider-plugin (v=${VERSION})"
+CGO_ENABLED=0 go build -ldflags="${LDFLAGS}" \
+    -o "${PLUGIN_DIR}/plexum-minimax-provider-plugin" \
+    ./cmd/plexum-minimax-provider-plugin
+
+cat > "${PLUGIN_DIR}/manifest.json" <<'EOF'
+{
+  "name": "plexum-minimax-provider",
+  "version": "0.1.0",
+  "activation": "lazy",
+  "executable": "plexum-minimax-provider-plugin",
+  "contracts": {
+    "provider": {
+      "models": ["MiniMax-M2.7", "MiniMax-M2.7-highspeed"]
+    }
+  }
+}
+EOF
+
+cat <<EOF
+
+[minimax-install] done.
+  plugin binary: ${PLUGIN_DIR}/plexum-minimax-provider-plugin
+  manifest:      ${PLUGIN_DIR}/manifest.json
+  models:        MiniMax-M2.7 / MiniMax-M2.7-highspeed
+  endpoint:      https://api.minimax.io/anthropic (default; CN via region:"cn")
+
+Next steps:
+  1. Write API key:
+       cat > ${PLUGIN_DIR}/config.json <<JSON
+       {"api_key": "sk-cp-..."}
+       JSON
+       chmod 600 ${PLUGIN_DIR}/config.json
+  2. Allow plugin:
+       Add "plexum-minimax-provider" to ${PROFILE_DIR}/plexum.json's
+       .plugins.allow array
+  3. Point an agent at MiniMax:
+       plexum agent-add --model MiniMax-M2.7 <agent-id>
+  4. systemctl --user restart plexum
+EOF