feat: add list_sessions top-level function

Ports listSessions() from the TypeScript SDK as a filesystem-reading
implementation (not a CLI passthrough). Scans ~/.claude/projects/ for
.jsonl session files and extracts metadata via stat + head/tail reads
without full JSONL parsing.

- Add SDKSessionInfo dataclass to types.py (session_id, summary,
  last_modified, file_size, custom_title, first_prompt, git_branch, cwd)
- Add _internal/sessions.py with path sanitization, JSON field
  extraction, first-prompt parsing, and git worktree detection
- Export list_sessions() and SDKSessionInfo from __init__.py
- Filter sidechain sessions and empty metadata-only sessions
  (no '(session)' placeholder fallback)
- Support include_worktrees option (default True) to scan all
  git worktree paths for a project
- 38 tests covering helpers and integration scenarios

Author: qing-ant

src/claude_agent_sdk/__init__.py

@@ -13,6 +13,7 @@
     CLINotFoundError,
     ProcessError,
 )
+from ._internal.sessions import list_sessions
 from ._internal.transport import Transport
 from ._version import __version__
 from .client import ClaudeSDKClient
@@ -52,6 +53,7 @@
     SandboxSettings,
     SdkBeta,
     SdkPluginConfig,
+    SDKSessionInfo,
     SettingSource,
     StopHookInput,
     SubagentStartHookInput,
@@ -378,6 +380,9 @@ async def call_tool(name: str, arguments: dict[str, Any]) -> Any:
     "SettingSource",
     # Plugin support
     "SdkPluginConfig",
+    # Session listing
+    "list_sessions",
+    "SDKSessionInfo",
     # Beta support
     "SdkBeta",
     # Sandbox support