feat: add list_sessions and get_session_messages top-level functions (#622)

Adds `list_sessions()` and `get_session_messages()` as top-level
functions, matching the TypeScript SDK's `listSessions()` and
`getSessionMessages()`.

These are **filesystem-reading implementations** (not CLI passthroughs)
— they scan `~/.claude/projects/` directly, same as the TS SDK.

## `list_sessions(directory=None, limit=None, include_worktrees=True) ->
list[SDKSessionInfo]`
- Scans project directories under `~/.claude/projects/` (or
`CLAUDE_CONFIG_DIR`)
- Reads head/tail 64KB of each `.jsonl` file (does not load entire large
transcripts)
- Extracts `session_id`, `summary`, `last_modified`, `file_size`,
`custom_title`, `first_prompt`, `git_branch`, `cwd`
- `include_worktrees=True` expands to active git worktree session dirs;
`False` filters them out
- Metadata-only sessions are dropped (no `'(session)'` placeholder)

## `get_session_messages(session_id, directory=None, limit=None,
offset=0) -> list[SessionMessage]`
- Reads one session's `.jsonl` fully
- Reconstructs the conversation chain via `parentUuid` backward
traversal (leaf → root, reversed)
- Handles branched transcripts (sidechains, edit history) by picking the
main-line leaf (non-sidechain, non-meta, highest file position)
- Filters to visible `user`/`assistant` messages only
- Returns `[]` for nonexistent/invalid session IDs (never raises)

<!-- CHANGELOG:START -->
- Add `list_sessions()` and `get_session_messages()` top-level functions
- Add `SDKSessionInfo` and `SessionMessage` dataclasses
<!-- CHANGELOG:END -->

## Test plan
- Unit: 59 tests covering path sanitization, JSONL head/tail parsing,
parentUuid chain reconstruction, main-chain leaf selection, visibility
filtering, limit/offset pagination
- E2E `list_sessions`: verified against 637 real sessions — all shape
checks pass, `include_worktrees` expansion confirmed on a 16-worktree
repo (270 → 274 sessions), nonexistent dir → clean `[]`
- E2E `get_session_messages`: 8 real sessions (~14MB JSONL) — chain
reconstruction proven (0 broken links across 1,581 entries), 254-branch
session correctly walks main-line, isMeta filtering verified, pagination
matches exact slicing

Author: qing-ant

src/claude_agent_sdk/__init__.py

@@ -13,6 +13,7 @@
     CLINotFoundError,
     ProcessError,
 )
+from ._internal.sessions import get_session_messages, list_sessions
 from ._internal.transport import Transport
 from ._version import __version__
 from .client import ClaudeSDKClient
@@ -59,6 +60,8 @@
     SandboxSettings,
     SdkBeta,
     SdkPluginConfig,
+    SDKSessionInfo,
+    SessionMessage,
     SettingSource,
     StopHookInput,
     SubagentStartHookInput,
@@ -402,6 +405,11 @@ async def call_tool(name: str, arguments: dict[str, Any]) -> Any:
     "SettingSource",
     # Plugin support
     "SdkPluginConfig",
+    # Session listing
+    "list_sessions",
+    "get_session_messages",
+    "SDKSessionInfo",
+    "SessionMessage",
     # Beta support
     "SdkBeta",
     # Sandbox support