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.

internal/runner/mcp_register.go

@@ -0,0 +1,70 @@
+// MCP server registration. gemini's `gemini mcp add -s user` writes
+// into the user's settings.json; we use a per-agent stable name so
+// multiple Plexum agents using gemini don't collide. Registration is
+// idempotent and performed lazily before the first turn that needs it.
+
+package runner
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"sync"
+)
+
+var registered sync.Map // agentID → bool (registration done this process)
+
+// EnsureGeminiMCPRegistered makes sure gemini's user settings has an
+// MCP server entry for this agent named plexum-host-<sanitized>.
+// Socket path is the stable StableSocketPath(agentID); same path the
+// per-turn bridge listens on.
+func EnsureGeminiMCPRegistered(ctx context.Context, geminiBinary, pluginBinary, agentID string) (string, error) {
+	name := geminiServerName(agentID)
+	if _, ok := registered.Load(name); ok {
+		return name, nil
+	}
+	// Remove first so we don't double-register; ignore error.
+	_ = exec.CommandContext(ctx, geminiBinary, "mcp", "remove", "-s", "user", name).Run()
+
+	args := []string{
+		"mcp", "add",
+		"-s", "user", "--trust",
+		"-e", "PLEXUM_MCP_SOCKET=" + StableSocketPath(agentID),
+		"-e", "PLEXUM_MCP_AGENT_ID=" + agentID,
+		name, pluginBinary, "mcp-host",
+	}
+	cmd := exec.CommandContext(ctx, geminiBinary, args...)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return "", fmt.Errorf("gemini mcp add: %w: %s", err, strings.TrimSpace(string(out)))
+	}
+	registered.Store(name, true)
+	return name, nil
+}
+
+// StableSocketPath returns the per-agent unix socket path the bridge
+// listens on AND gemini's mcp-host subprocess dials into.
+func StableSocketPath(agentID string) string {
+	return filepath.Join(os.TempDir(), "plexum-gemini-mcp-"+sanitize(agentID)+".sock")
+}
+
+func geminiServerName(agentID string) string {
+	return "plexum-host-" + sanitize(agentID)
+}
+
+func sanitize(s string) string {
+	out := make([]byte, 0, len(s))
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		switch {
+		case c >= 'a' && c <= 'z', c >= 'A' && c <= 'Z',
+			c >= '0' && c <= '9', c == '-', c == '_':
+			out = append(out, c)
+		default:
+			out = append(out, '_')
+		}
+	}
+	return string(out)
+}