feat(gemini): plexum-host MCP exposure + real consume mirror

Same architecture as the codex sister change:

1. mcp-host subcommand routes argv[1] to mcpbridge.Run, reading
   PLEXUM_MCP_SOCKET / PLEXUM_MCP_AGENT_ID from gemini-cli's
   `gemini mcp add --env` env baking.

2. EnsureGeminiMCPRegistered handles per-agent stable registration
   under `-s user --trust` so gemini auto-approves the tool surface.

3. Post-turn ParseGeminiToolCalls + EmitGeminiToolCalls scans the
   chat JSONL at ~/.gemini/tmp/<ws>/chats/session-*.jsonl. Pairs
   gemini's nested `toolCalls[].id` with the matching
   `functionResponse.id` from later user lines.

4. MutateGeminiSession rewrites the chat JSONL: toolCalls[].args
   → {} (heavy), functionResponse.response.output → marker.

E2E verified: gemini call exec via plexum-host → dynamic.jsonl
records the call → dynamic-tool-clear consumes → gemini session
mirrored → resume gemini sees consumed marker not the original.

cmd/plexum-gemini-provider-plugin/main.go

@@ -25,6 +25,7 @@ import (
 	"path/filepath"
 
 	"git.hangman-lab.top/hzhang/Plexum-sdk-go/canonical"
+	"git.hangman-lab.top/hzhang/Plexum-sdk-go/mcpbridge"
 	plugin "git.hangman-lab.top/hzhang/Plexum-sdk-go/plugin"
 
 	"git.hangman-lab.top/hzhang/Plexum-gemini-provider/internal/runner"
@@ -125,31 +126,29 @@ func (p *geminiPlugin) StreamWithAgent(ctx context.Context, modelID string, agen
 	return runner.Run(ctx, p.host, agent, cliModel, p.cfg.Binary, p.cfg.ExtraArgs, req)
 }
 
-// MutateSession is the session-mirror hook (decision #29x). gemini-cli
-// stores transcripts under ~/.gemini/tmp/<workspace-name>/chats/
-// session-<ISO>-<session_id_prefix>.jsonl with a `$set`-style mutation
-// log (each line either a session-meta header or `{"$set":{...}}`
-// applied in order). Tool-call records use gemini-cli's own id
-// namespace (no shape doc as of this writing) which doesn't match
-// Plexum's toolu_*-style block ids.
-//
-// v1 ships as a logged no-op: we resolve the file path so operator
-// telemetry confirms wiring, but we don't attempt to rewrite. When
-// Plexum starts routing tool-call ids through gemini's native surface
-// (preserving the round-trip), the rewriter plugs in here.
+// MutateSession mirrors host dynamic-* consume mutations onto the
+// gemini chat JSONL at ~/.gemini/tmp/<ws>/chats/session-*.jsonl so
+// the next `gemini --resume` sees the consumed view. Tool-call
+// round-trip is captured by runner.ParseGeminiToolCalls +
+// EmitGeminiToolCalls (post-turn), so the BlockMutations we receive
+// here resolve to ids actually in gemini's session.
 func (p *geminiPlugin) MutateSession(ctx context.Context, req plugin.SessionMutateRequest) error {
-	path, err := runner.FindGeminiSessionFile(req.Workspace)
-	if err != nil {
-		p.host.Log("debug", "gemini session mutate: find failed", map[string]any{
-			"agent": req.AgentID, "err": err.Error(),
-		})
+	if len(req.Mutations) == 0 {
 		return nil
 	}
-	p.host.Log("info", "gemini session mutate (no-op v1)", map[string]any{
-		"agent":        req.AgentID,
-		"session_path": path,
-		"requested":    len(req.Mutations),
-		"reason":       "gemini tool-call id round-trip not yet wired",
+	mu := make([]runner.GeminiMutation, 0, len(req.Mutations))
+	for _, m := range req.Mutations {
+		mu = append(mu, runner.GeminiMutation{BlockID: m.BlockID, Op: m.Op})
+	}
+	touched, err := runner.MutateGeminiSession(req.Workspace, mu)
+	if err != nil {
+		p.host.Log("warn", "gemini session mutate failed", map[string]any{
+			"agent": req.AgentID, "err": err.Error(),
+		})
+		return err
+	}
+	p.host.Log("info", "gemini session mutate complete", map[string]any{
+		"agent": req.AgentID, "touched": touched, "requested": len(req.Mutations),
 	})
 	return nil
 }
@@ -169,8 +168,41 @@ func mapModel(plexumModel string) string {
 }
 
 func main() {
+	if len(os.Args) > 1 && os.Args[1] == "mcp-host" {
+		if err := runMCPHost(os.Args[2:]); err != nil {
+			fmt.Fprintf(os.Stderr, "mcp-host: %v\n", err)
+			os.Exit(1)
+		}
+		return
+	}
 	if err := plugin.Serve(&geminiPlugin{}); err != nil {
 		fmt.Fprintf(os.Stderr, "plexum-gemini-provider-plugin: %v\n", err)
 		os.Exit(1)
 	}
 }
+
+func runMCPHost(argv []string) error {
+	opts := mcpbridge.Opts{
+		SocketPath: os.Getenv("PLEXUM_MCP_SOCKET"),
+		AgentID:    os.Getenv("PLEXUM_MCP_AGENT_ID"),
+	}
+	for i := 0; i < len(argv); i++ {
+		switch argv[i] {
+		case "--socket":
+			if i+1 >= len(argv) {
+				return errors.New("--socket needs value")
+			}
+			opts.SocketPath = argv[i+1]
+			i++
+		case "--agent-id":
+			if i+1 >= len(argv) {
+				return errors.New("--agent-id needs value")
+			}
+			opts.AgentID = argv[i+1]
+			i++
+		default:
+			return fmt.Errorf("unknown mcp-host flag: %s", argv[i])
+		}
+	}
+	return mcpbridge.Run(context.Background(), opts)
+}