feat: add /sessions command for session history management (#142)

Add a new /sessions command to manage Claude Code session history with
alias support for quick access to previous sessions.

Features:
- List sessions with pagination and filtering (by date, ID)
- Load and view session content and metadata
- Create memorable aliases for sessions
- Remove aliases
- Display session statistics (lines, items, size)
- List all aliases

New libraries:
- scripts/lib/session-manager.js - Core session CRUD operations
- scripts/lib/session-aliases.js - Alias management with atomic saves

New command:
- commands/sessions.md - Complete command with embedded scripts

Modified:
- scripts/lib/utils.js - Add getAliasesPath() export
- scripts/hooks/session-start.js - Show available aliases on session start

Session format support:
- Old: YYYY-MM-DD-session.tmp
- New: YYYY-MM-DD-<short-id>-session.tmp

Aliases are stored in ~/.claude/session-aliases.json with Windows-
compatible atomic writes and backup support.

Co-authored-by: 王志坚 <wangzhijian10@bgyfw.com>
Co-authored-by: Claude <noreply@anthropic.com>

scripts/hooks/session-start.js

@@ -16,6 +16,7 @@ const {
   log
 } = require('../lib/utils');
 const { getPackageManager, getSelectionPrompt } = require('../lib/package-manager');
+const { listAliases } = require('../lib/session-aliases');
 
 async function main() {
   const sessionsDir = getSessionsDir();
@@ -42,6 +43,15 @@ async function main() {
     log(`[SessionStart] ${learnedSkills.length} learned skill(s) available in ${learnedDir}`);
   }
 
+  // Check for available session aliases
+  const aliases = listAliases({ limit: 5 });
+
+  if (aliases.length > 0) {
+    const aliasNames = aliases.map(a => a.name).join(', ');
+    log(`[SessionStart] ${aliases.length} session alias(es) available: ${aliasNames}`);
+    log(`[SessionStart] Use /sessions load <alias> to continue a previous session`);
+  }
+
   // Detect and report package manager
   const pm = getPackageManager();
   log(`[SessionStart] Package manager: ${pm.name} (${pm.source})`);