docs: document SQLite session cleanup

[00200200]

Summary

Document the lifecycle of the SQLiteSession used in the session quickstart. The example now calls session.close() and explains that cleanup is synchronous, releases database connections and file locks, and makes the session unusable afterward.

Test plan

• git diff --check
• make build-docs (completed successfully; the existing build emits unrelated duplicate-reference warnings)

Issue number

N/A

Checks

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