fix(pi): point pi at config dir via PI_CODING_AGENT_DIR instead of redirecting HOME

[dgokeeffe]

Problem

Pi redirected $HOME to APP_DIR/pi-home to isolate its configuration. On macOS that also changed default keychain resolution, causing gh auth, git credential helpers, and other keychain-backed tools to fail inside Pi.

Fix

Set Pi's supported PI_CODING_AGENT_DIR environment variable instead of replacing HOME:

 env["OAUTH_TOKEN"] = token
-env["HOME"] = str(PI_UCODE_HOME)
+env["PI_CODING_AGENT_DIR"] = str(PI_CONFIG_DIR)

Pi continues to store models.json, settings.json, and sessions under the existing ucode-private agent directory, while subprocesses keep the real user home and can resolve the login keychain normally.

Test changes

• Redirect PI_CONFIG_DIR, models, settings, and backup paths consistently in both Pi E2E fixtures.
• Assert build_runtime_env() sets PI_CODING_AGENT_DIR and preserves an existing HOME.
• Update stale comments/docstrings that described the old HOME-based behavior.

Validation

• uv run ruff check src/ucode/agents/pi.py tests/test_agent_pi.py tests/test_e2e.py tests/test_e2e_user_agent.py
• uv run pytest tests/test_agent_pi.py tests/test_e2e_user_agent.py::TestPiUserAgent tests/test_e2e.py -q — 42 passed, 29 workspace-gated tests skipped

This remains an independent one-commit, four-file fix based on current main.

[dgokeeffe]

Rebased onto current main and tightened after review: the test now explicitly proves HOME is preserved, stale HOME-based comments/docstrings were updated, and focused validation is 42 passed / 29 workspace-gated skips with Ruff clean. A maintainer CI approval/review is now the remaining gate.

[dgokeeffe]

Rebased on current main. All four PRs in this stack are now 0 commits behind main and report MERGEABLE.

The remaining blocker is outside my control. Because these are fork PRs, every CI run has ended in action_required — 21 runs since 2026-07-16, not one has ever executed. The active main protection ruleset requires the test and e2e checks to pass, so all four PRs stay BLOCKED until someone with write access clicks Approve and run workflows. I also can't add reviewers myself (the request-reviewers API 404s without push access), which is why none of these has a reviewer assigned.

@asujithan @rohita5l — could one of you approve the workflow runs?

I'd suggest starting with this PR: it's the smallest of the four (+17 / −7, 4 files). It stops ucode from redirecting HOME when launching Pi and points Pi at its config dir via PI_CODING_AGENT_DIR instead. Local validation on this branch: ruff clean, 1046 passed / 6 skipped.

One heads-up on the e2e gate: fork PRs receive empty secrets, so UCODE_TEST_WORKSPACE is blank and the workspace-gated tests skip via tests/conftest.py:46. The job will go green without exercising a live workspace — worth knowing when you read the check result.

Happy to take the other three (#223, #239, #217) one at a time after this one lands.