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/lib/utils.js

@@ -34,6 +34,13 @@ function getSessionsDir() {
   return path.join(getClaudeDir(), 'sessions');
 }
 
+/**
+ * Get the session aliases file path
+ */
+function getAliasesPath() {
+  return path.join(getClaudeDir(), 'session-aliases.json');
+}
+
 /**
  * Get the learned skills directory
  */
@@ -382,6 +389,7 @@ module.exports = {
   getHomeDir,
   getClaudeDir,
   getSessionsDir,
+  getAliasesPath,
   getLearnedSkillsDir,
   getTempDir,
   ensureDir,