Move log and history files to $XDG_STATE_HOME

[jrraymond]

Fixes #1497.

Summary

Per the XDG Base Directory Specification, $XDG_STATE_HOME (default ~/.local/state) is the correct location for state data that persists across restarts but is not user-configuration. Log and history fit this definition; config and casing remain in $XDG_CONFIG_HOME.

• Add state_location() to config.py, mirroring config_location() but using $XDG_STATE_HOME (defaults to ~/.local/state/pgcli/). On Windows, returns the same path as config_location() since there is no XDG equivalent.
• Add migrate_file(old, new) helper that silently moves a file from the old location to the new one on first launch after upgrade. Errors are logged rather than raised.
• initialize_logging() and run_cli() now use state_location() for the default log and history paths, and call migrate_file() to handle the one-time upgrade migration.
• Update default path comments in pgclirc.
• Add tests for state_location() and all migration cases (old only, both exist, old missing, error logged on failure).

Testing

> pgcli
Server: PostgreSQL 18.3 (Homebrew)
Version: 4.4.0
Home: http://pgcli.com
justinraymond> select * from foo;
Goodbye!

# logs/history resepect XDG_STATE_HOME
> cat ~/.local/state/pgcli/history

# 2026-04-03 11:57:59.545613
+select * from foo;

> cat ~/.local/state/pgcli/log
2026-04-03 11:51:19,816 (77436/MainThread) pgcli.main INFO - No default pager found in environment. Using os default pager
...

# move history/logs to old location to test migration
> mv ~/.local/state/pgcli/* ~/.config/pgcli/
> rm -r ~/.local/state/pgcli/

> pgcli
Server: PostgreSQL 18.3 (Homebrew)
Version: 4.4.0
Home: http://pgcli.com
justinraymond> select * from bar;
Goodbye!

> cat ~/.local/state/pgcli/history

# 2026-04-03 11:57:59.545613
+select * from foo;

# 2026-04-03 11:59:40.069136
+select * from bar;

> cat ~/.local/state/pgcli/log
2026-04-03 11:51:19,816 (77436/MainThread) pgcli.main INFO - No default pager found in environment. Using os default pager
...

# old location empty
> tree ~/.config/pgcli/
/Users/justinraymond/.config/pgcli/
└── config

1 directory, 1 file

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install).
• I verified that my changes work as expected.
• Please squash merge this pull request.

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