feat: support RDT_CONFIG_DIR and XDG_CONFIG_HOME for config path override

[cestivan]

Problem

The config directory is currently hardcoded to ~/.config/rdt-cli:

CONFIG_DIR = Path.home() / ".config" / "rdt-cli"

This causes two issues:

1. Multi-instance isolation — when multiple independent processes (e.g. automated agents, CI jobs, or per-user instances) run on the same machine, they all read/write the same credential file. This leads to credential collisions and unintended sharing.
2. XDG non-compliance — the XDG Base Directory spec is the standard way for users to relocate config on Linux/macOS, but rdt-cli ignores XDG_CONFIG_HOME.

Solution

Resolve the config directory using a priority chain with no breaking change:

Priority	Source	Example
1	RDT_CONFIG_DIR env var	RDT_CONFIG_DIR=/tmp/myagent/rdt rdt whoami
2	XDG_CONFIG_HOME/rdt-cli	XDG_CONFIG_HOME=~/.config-work rdt whoami
3	~/.config/rdt-cli	existing default — unchanged

The implementation is a small helper function in constants.py:

def _resolve_config_dir() -> Path:
    if rdt_dir := os.environ.get("RDT_CONFIG_DIR"):
        return Path(rdt_dir)
    if xdg_home := os.environ.get("XDG_CONFIG_HOME"):
        return Path(xdg_home) / "rdt-cli"
    return Path.home() / ".config" / "rdt-cli"

Changes

• rdt_cli/constants.py — introduce _resolve_config_dir(), replace the hardcoded path
• README.md — document RDT_CONFIG_DIR and XDG_CONFIG_HOME in the Environment Variables table; link the Authentication section to it

Backwards compatibility

No breaking change. When neither env var is set, the resolved path is identical to the previous hardcoded value.