feat(appstash): add APPSTASH_BASE_DIR env var support

[pyramation]

Summary

Adds APPSTASH_BASE_DIR environment variable support to the appstash() directory resolution chain. When no baseDir option is passed programmatically, appstash now checks process.env.APPSTASH_BASE_DIR before falling back to os.homedir().

Resolution priority: baseDir option > APPSTASH_BASE_DIR env var > os.homedir() > XDG fallback > tmp

This enables test isolation for generated CLIs (which call createConfigStore(toolName) without a baseDir option) without overriding HOME.

Review & Testing Checklist for Human

• Verify the env var name APPSTASH_BASE_DIR doesn't conflict with anything already in use across your projects (grep -r APPSTASH_BASE_DIR across repos)
• Confirm this doesn't unintentionally affect production CLI behavior — the env var would need to be explicitly set, but worth checking CI environments and Dockerfiles for any pre-existing use

Notes

• An empty string for APPSTASH_BASE_DIR is falsy and correctly falls through to os.homedir()
• Motivation: CLI e2e tests in constructive currently override HOME to isolate appstash config; this env var provides a cleaner alternative

Link to Devin session: https://app.devin.ai/sessions/c92c3a11450342f8875625a60fa1be28
Requested by: @pyramation

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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