feat(session): integrate JSONL persistence into agent loop (#1170)

* feat(session): add SessionStore interface and JSONL backend adapter

Extract a SessionStore interface from the methods the agent loop uses
(AddMessage, GetHistory, SetSummary, TruncateHistory, Save, etc.).
Both SessionManager and the new JSONLBackend satisfy this interface,
allowing the persistence layer to be swapped transparently.

JSONLBackend wraps memory.Store and maps its error-returning API to
the fire-and-forget contract that the agent loop expects — write
errors are logged, reads return empty defaults on failure. Save()
triggers compaction to reclaim space after logical truncation.

Part of #1169

* test(session): add JSONLBackend integration tests

8 tests covering the full SessionStore contract through the JSONL
backend: message roundtrip, tool calls, summary, truncation with
compaction, history replacement, empty sessions, session isolation,
and the complete summarization flow (SetSummary → TruncateHistory →
Save).

Includes compile-time interface satisfaction checks for both
SessionManager and JSONLBackend.

Part of #1169

* feat(agent): wire JSONL session store into agent loop

Replace the concrete *SessionManager field with the SessionStore
interface and initialize the JSONL backend by default. Legacy .json
session files are auto-migrated on first startup. Falls back to
SessionManager if the JSONL store cannot be initialized.

The agent loop code (loop.go) requires zero changes — all method
calls work identically through the interface.

Closes #1169

* fix(session): propagate compact error from Save

Save() was swallowing the error returned by Compact and always
returning nil. Callers checking Save's return value would never
see a compaction failure. Return the error directly so the agent
loop can log or handle it as needed.

* feat(session): add Close to SessionStore interface

Add Close() error to SessionStore so callers can release resources
through the interface. JSONLBackend already had Close; this adds
a no-op implementation to SessionManager for compatibility.

* fix(session): close session stores on shutdown and harden migration

- Add Close() to AgentInstance, AgentRegistry, and AgentLoop so JSONL
  file handles are released during gateway shutdown and CLI exit.
- Fall back to SessionManager when migration fails, preventing a split
  state where some sessions live in JSONL and others remain in JSON.
- Add defer agentLoop.Close() in the CLI agent command path.
- Document SessionStore interface methods (fire-and-forget contract).

pkg/agent/instance.go

@@ -1,6 +1,7 @@
 package agent
 
 import (
+	"context"
 	"fmt"
 	"log"
 	"os"
@@ -9,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/sipeed/picoclaw/pkg/config"
+	"github.com/sipeed/picoclaw/pkg/memory"
 	"github.com/sipeed/picoclaw/pkg/providers"
 	"github.com/sipeed/picoclaw/pkg/routing"
 	"github.com/sipeed/picoclaw/pkg/session"
@@ -31,7 +33,7 @@ type AgentInstance struct {
 	SummarizeMessageThreshold int
 	SummarizeTokenPercent     int
 	Provider                  providers.LLMProvider
-	Sessions                  *session.SessionManager
+	Sessions                  session.SessionStore
 	ContextBuilder            *ContextBuilder
 	Tools                     *tools.ToolRegistry
 	Subagents                 *config.SubagentsConfig
@@ -95,7 +97,7 @@ func NewAgentInstance(
 	}
 
 	sessionsDir := filepath.Join(workspace, "sessions")
-	sessionsManager := session.NewSessionManager(sessionsDir)
+	sessions := initSessionStore(sessionsDir)
 
 	mcpDiscoveryActive := cfg.Tools.MCP.Enabled && cfg.Tools.MCP.Discovery.Enabled
 	contextBuilder := NewContextBuilder(workspace).WithToolDiscovery(
@@ -226,7 +228,7 @@ func NewAgentInstance(
 		SummarizeMessageThreshold: summarizeMessageThreshold,
 		SummarizeTokenPercent:     summarizeTokenPercent,
 		Provider:                  provider,
-		Sessions:                  sessionsManager,
+		Sessions:                  sessions,
 		ContextBuilder:            contextBuilder,
 		Tools:                     toolsRegistry,
 		Subagents:                 subagents,
@@ -280,6 +282,39 @@ func compilePatterns(patterns []string) []*regexp.Regexp {
 	return compiled
 }
 
+// Close releases resources held by the agent's session store.
+func (a *AgentInstance) Close() error {
+	if a.Sessions != nil {
+		return a.Sessions.Close()
+	}
+	return nil
+}
+
+// initSessionStore creates the session persistence backend.
+// It uses the JSONL store by default and auto-migrates legacy JSON sessions.
+// Falls back to SessionManager if the JSONL store cannot be initialized or
+// if migration fails (which indicates the store cannot write reliably).
+func initSessionStore(dir string) session.SessionStore {
+	store, err := memory.NewJSONLStore(dir)
+	if err != nil {
+		log.Printf("memory: init store: %v; using json sessions", err)
+		return session.NewSessionManager(dir)
+	}
+
+	if n, merr := memory.MigrateFromJSON(context.Background(), dir, store); merr != nil {
+		// Migration failure means the store could not write data.
+		// Fall back to SessionManager to avoid a split state where
+		// some sessions are in JSONL and others remain in JSON.
+		log.Printf("memory: migration failed: %v; falling back to json sessions", merr)
+		store.Close()
+		return session.NewSessionManager(dir)
+	} else if n > 0 {
+		log.Printf("memory: migrated %d session(s) to jsonl", n)
+	}
+
+	return session.NewJSONLBackend(store)
+}
+
 func expandHome(path string) string {
 	if path == "" {
 		return path