feat(gemini): SessionMutator stub + session-file resolver

Wires the host's mirror hook end-to-end: FindGeminiSessionFile resolves
~/.gemini/tmp/<workspace>/chats/session-<ISO>-<sid_prefix>.jsonl from
the session id captured into .plexum-gemini-session, picking the
most-recent match by mtime. Logs the no-op for telemetry.

Real rewrite is deferred to v2: gemini-cli's tool-call id namespace
doesn't match Plexum's canonical block format (and its on-disk shape
is `$set`-style mutation log rather than flat append-only). When
tool-call round-trip through gemini's native surface lands, the
rewriter plugs into this file's path resolver.

internal/runner/session_mutate.go

@@ -0,0 +1,91 @@
+// session_mutate.go — gemini session file path resolution. The actual
+// JSONL rewrite is deferred (see plugin main.go MutateSession comment):
+// gemini-cli uses its own tool-call id namespace that doesn't match
+// Plexum's, so v1 ships as a logged no-op. This file owns the path
+// resolver so the v2 rewriter has its entry point.
+//
+// Layout (observed):
+//
+//	~/.gemini/tmp/<workspace-basename>/chats/session-<ISO>-<sid>.jsonl
+//
+// where <sid> is the first 8 chars of the session_id captured into
+// workspace/.plexum-gemini-session. We pick the file whose name
+// contains that prefix; multiple matches → the most-recent by mtime.
+
+package runner
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+)
+
+// FindGeminiSessionFile resolves the chat JSONL gemini-cli writes for
+// the session id captured in workspace/.plexum-gemini-session.
+// Returns ("", error) if no session id is recorded yet OR no chat
+// file is on disk.
+func FindGeminiSessionFile(workspace string) (string, error) {
+	sid := loadSessionID(workspace)
+	if sid == "" {
+		return "", errors.New("no gemini session id captured yet")
+	}
+	prefix := sid
+	if len(prefix) > 8 {
+		prefix = prefix[:8]
+	}
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	root := filepath.Join(home, ".gemini", "tmp")
+	wsName := filepath.Base(workspace)
+	candidates := []string{
+		filepath.Join(root, wsName, "chats"),
+		// Fallback dirs (older / alternate gemini-cli versions). We
+		// glob defensively so an operator's custom layout still
+		// surfaces something for telemetry.
+	}
+	entries := []os.DirEntry{}
+	chosenDir := ""
+	for _, dir := range candidates {
+		es, err := os.ReadDir(dir)
+		if err == nil {
+			entries = es
+			chosenDir = dir
+			break
+		}
+	}
+	if chosenDir == "" {
+		return "", fmt.Errorf("no gemini chats dir under %s", root)
+	}
+	// Pick the newest session-*.jsonl whose name contains the sid prefix.
+	type cand struct {
+		path  string
+		mtime int64
+	}
+	var matches []cand
+	for _, e := range entries {
+		if e.IsDir() || !strings.HasSuffix(e.Name(), ".jsonl") {
+			continue
+		}
+		if !strings.Contains(e.Name(), prefix) {
+			continue
+		}
+		info, err := e.Info()
+		if err != nil {
+			continue
+		}
+		matches = append(matches, cand{
+			path:  filepath.Join(chosenDir, e.Name()),
+			mtime: info.ModTime().UnixNano(),
+		})
+	}
+	if len(matches) == 0 {
+		return "", fmt.Errorf("no chat file matching session_id prefix %q in %s", prefix, chosenDir)
+	}
+	sort.Slice(matches, func(i, j int) bool { return matches[i].mtime > matches[j].mtime })
+	return matches[0].path, nil
+}