fix: resolve multiple bugs from code review #116

Fixes four issues identified in the community code review:

- Session persistence broken on Windows: session keys like
  "telegram:123456" contain ':', which is illegal in Windows
  filenames. filepath.Base() strips drive-letter prefixes on Windows,
  causing Save() to silently fail. Added sanitizeFilename() to
  replace invalid chars in the filename while keeping the original
  key in the JSON payload.

- HTTP client with no timeout: HTTPProvider used Timeout: 0 (infinite
  wait), which can hang the entire agent if an API endpoint becomes
  unresponsive. Set a 120s safety timeout.

- Slack AllowFrom type mismatch: SlackConfig used plain []string
  while every other channel uses FlexibleStringSlice, so numeric
  user IDs in Slack config would fail to parse.

- Token estimation wrong for CJK: estimateTokens() divided byte
  length by 4, but CJK characters are 3 bytes each, causing ~3x
  overestimation and premature summarization. Switched to
  utf8.RuneCountInString() / 3 for better cross-language accuracy.

Also added unit tests for the session filename sanitization.

Ref #116

pkg/session/manager_test.go

@@ -0,0 +1,74 @@
+package session
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestSanitizeFilename(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+	}{
+		{"simple", "simple"},
+		{"telegram:123456", "telegram_123456"},
+		{"discord:987654321", "discord_987654321"},
+		{"slack:C01234", "slack_C01234"},
+		{"no-colons-here", "no-colons-here"},
+		{"multiple:colons:here", "multiple_colons_here"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			got := sanitizeFilename(tt.input)
+			if got != tt.expected {
+				t.Errorf("sanitizeFilename(%q) = %q, want %q", tt.input, got, tt.expected)
+			}
+		})
+	}
+}
+
+func TestSave_WithColonInKey(t *testing.T) {
+	tmpDir := t.TempDir()
+	sm := NewSessionManager(tmpDir)
+
+	// Create a session with a key containing colon (typical channel session key).
+	key := "telegram:123456"
+	sm.GetOrCreate(key)
+	sm.AddMessage(key, "user", "hello")
+
+	// Save should succeed even though the key contains ':'
+	if err := sm.Save(key); err != nil {
+		t.Fatalf("Save(%q) failed: %v", key, err)
+	}
+
+	// The file on disk should use sanitized name.
+	expectedFile := filepath.Join(tmpDir, "telegram_123456.json")
+	if _, err := os.Stat(expectedFile); os.IsNotExist(err) {
+		t.Fatalf("expected session file %s to exist", expectedFile)
+	}
+
+	// Load into a fresh manager and verify the session round-trips.
+	sm2 := NewSessionManager(tmpDir)
+	history := sm2.GetHistory(key)
+	if len(history) != 1 {
+		t.Fatalf("expected 1 message after reload, got %d", len(history))
+	}
+	if history[0].Content != "hello" {
+		t.Errorf("expected message content %q, got %q", "hello", history[0].Content)
+	}
+}
+
+func TestSave_RejectsPathTraversal(t *testing.T) {
+	tmpDir := t.TempDir()
+	sm := NewSessionManager(tmpDir)
+
+	badKeys := []string{"", ".", "..", "foo/bar", "foo\\bar"}
+	for _, key := range badKeys {
+		sm.GetOrCreate(key)
+		if err := sm.Save(key); err == nil {
+			t.Errorf("Save(%q) should have failed but didn't", key)
+		}
+	}
+}