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.

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

@@ -125,6 +125,35 @@ 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.
+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(),
+		})
+		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",
+	})
+	return nil
+}
+
 // mapModel converts a Plexum-advertised model id into the CLI's
 // --model value. Returns "" for "gemini" (use CLI default) and
 // passes any other unrecognized id through verbatim (operator may