feat: Support configurable home directory via TAU_HOME environment variable

[abrahamy]

Summary

Adds support for TAU_HOME and TAU_AGENTS_HOME environment variables to override the default ~/.tau and ~/.agents directories respectively.

Closes #465

Changes

• Add _default_tau_home() and _default_agents_home() helper functions in src/tau_coding/paths.py
• Update TauPaths dataclass to use the new helper functions as defaults
• Add comprehensive test coverage for the new functionality (5 new tests through public API)
• Update documentation to describe the new environment variables

Motivation

Tau currently hardcodes the user home directory to ~/.tau. This prevents users from:

• Storing Tau configuration in XDG-compliant locations (e.g., ~/.config/tau)
• Using different Tau home directories for different projects or contexts
• Following organizational policies that require non-standard configuration paths
• Managing dotfiles with tools like GNU Stow where ~/.tau isn't ideal

Implementation Details

The implementation:

• Respects TAU_HOME when set, falls back to ~/.tau otherwise
• Respects TAU_AGENTS_HOME when set, falls back to ~/.agents otherwise
• Supports tilde expansion (e.g., ~/my-tau)
• Maintains full backward compatibility

def _default_tau_home() -> Path:
    """Return the Tau home directory, respecting TAU_HOME if set."""
    if tau_home := environ.get("TAU_HOME"):
        return Path(tau_home).expanduser()
    return Path.home() / ".tau"

Testing

All existing tests pass, plus 5 new tests covering behavior through the public TauPaths API:

• test_taupaths_defaults_to_standard_locations - Verifies default ~/.tau and ~/.agents when env vars are not set
• test_taupaths_respects_tau_home_env_var - Verifies TAU_HOME override works
• test_taupaths_respects_tau_agents_home_env_var - Verifies TAU_AGENTS_HOME override works
• test_taupaths_expands_tilde_in_tau_home - Verifies tilde expansion for TAU_HOME
• test_taupaths_expands_tilde_in_tau_agents_home - Verifies tilde expansion for TAU_AGENTS_HOME

uv run pytest tests/test_paths.py -v
# 8 passed in 0.46s

uv run pytest
# 1150 passed in 69.18s

Documentation

Updated website/content/reference/configuration.md to document:

• The new TAU_HOME environment variable
• The new TAU_AGENTS_HOME environment variable
• Example usage for XDG-compliant locations

Checks

• uv run ruff check . - passed
• uv run ruff format --check . - passed
• uv run mypy - passed
• uv run pytest - all 1150 tests passed