feat: support TRACHE_HOME env var for shared cache directory

[antitree]

Summary

• Adds support for a TRACHE_HOME environment variable that overrides the default .trache directory lookup
• When TRACHE_HOME is set, trache uses that path as its root regardless of the current working directory
• Falls back to .trache in cwd when unset — fully backwards compatible, no behaviour change for existing users

Motivation

Currently trache always looks for .trache/ relative to the current working directory. This means users who run trache from multiple locations (different project dirs, scripts, AI agent sessions) need a separate .trache/ copy in each one, or must always cd to the same directory first.

Setting TRACHE_HOME to a fixed path (e.g. in ~/.zshrc) solves this cleanly:

export TRACHE_HOME="$HOME/projects/my-trache-store/.trache"

Change

One-line change in cli/_context.py:

# Before
TRACHE_ROOT = Path(".trache")

# After
TRACHE_ROOT = Path(os.environ["TRACHE_HOME"]) if "TRACHE_HOME" in os.environ else Path(".trache")

Test plan

• With TRACHE_HOME unset: existing behaviour unchanged, .trache/ resolved from cwd
• With TRACHE_HOME set to an absolute path: trache board list, trache card list, trache status all work correctly from any directory
• Tested manually from /tmp with TRACHE_HOME pointing at an existing cache — boards and cards resolved correctly

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