Keep SQLite sessions on the original database

[1fanwang]

Summary

After changing directories, a SQLite session created with a relative filename can read or clear another database, or raise sqlite3.OperationalError: no such table: agent_messages.

Bind the filename at construction. In-memory sessions and literal ~ filenames keep their behavior.

Test plan

Ran this against both versions with real SQLite files.

Source	Command	Result
Base	Inline program below	Missing-table error
Fix	Same program	Original database retained

Reproducer and raw logs

PYTHONPATH=src python - <<'PY'
import asyncio
import os
import sqlite3
from contextlib import closing
from tempfile import TemporaryDirectory

from agents import SQLiteSession

original_cwd: str = os.getcwd()
try:
    with TemporaryDirectory() as root:
        try:
            os.chdir(root)
            os.mkdir("other")
            with closing(SQLiteSession(session_id="example", db_path="history.db")) as session:
                os.chdir("other")
                print("history:", asyncio.run(session.get_items()))
                print("second database created:", os.path.exists("history.db"))
        finally:
            os.chdir(original_cwd)
except sqlite3.OperationalError as error:
    print(f"sqlite3.OperationalError: {error}")
    raise SystemExit(1) from None
PY

Before:

sqlite3.OperationalError: no such table: agent_messages
exit=1

After:

history: []
second database created: False
exit=0

The full check has one unrelated docs-publisher failure on both versions: Git's safe.bareRepository=explicit rejects its bare-repo fixture. The SQLite and serial tests pass.

Issue number

No issue filed.

Checks

• I've added new tests, if relevant
• I've run .agents/skills/code-change-verification/scripts/run.sh
• I've confirmed all verification steps pass (Docs fixture failure above.)
• If using Codex, I've run /review before submitting this PR (Not applicable.)