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 config (~/.pi/agent), but this broke macOS keychain default resolution under ucode. The Security framework looks for the login keychain under the redirected HOME, finds none, and security default-keychain returns "A default keychain could not be found". As a result gh auth, the git credential helper, and any keychain-backed tool failed inside pi.

Root cause

Pi does honor a config-dir env var. From the installed package (config.js):

export const ENV_AGENT_DIR = `${APP_NAME.toUpperCase()}_CODING_AGENT_DIR`; // → PI_CODING_AGENT_DIR
export function getAgentDir() {
    const envDir = process.env[ENV_AGENT_DIR];
    if (envDir) return expandTildePath(envDir);          // env wins
    return join(homedir(), CONFIG_DIR_NAME, "agent");     // else ~/.pi/agent
}

So the $HOME redirect was unnecessary — ucode can isolate pi's config the same way it isolates opencode (XDG_CONFIG_HOME) and gemini (GEMINI_CLI_HOME): by pointing the agent at a config dir via env var, without touching $HOME.

Fix

build_runtime_env now sets PI_CODING_AGENT_DIR = PI_CONFIG_DIR instead of HOME = PI_UCODE_HOME:

 def build_runtime_env(token: str) -> dict[str, str]:
     env = os.environ.copy()
     env["OAUTH_TOKEN"] = token
-    env["HOME"] = str(PI_UCODE_HOME)
+    env["PI_CODING_AGENT_DIR"] = str(PI_CONFIG_DIR)
     return env

Config isolation is preserved — models.json, settings.json, and sessions still land under APP_DIR/pi-home/.pi/agent (the existing PI_CONFIG_DIR). But $HOME stays the user's real home, so the login keychain stays discoverable and gh/git credential helper/keychain-backed tools work unmodified.

Test changes

The two pi e2e sites monkeypatched PI_UCODE_HOME/PI_CONFIG_PATH to redirect pi at a tmp home. They now also patch:

• PI_CONFIG_DIR — read by build_runtime_env to set the env var
• PI_SETTINGS_PATH / PI_SETTINGS_BACKUP_PATH — previously masked because the HOME redirect made pi read settings from the un-patched real APP_DIR path; without the redirect, _write_settings would write to the real path while pi reads from the tmp PI_CONFIG_DIR

The pi unit test assertion (env["HOME"] == ... → env["PI_CODING_AGENT_DIR"] == ...) is updated to match.

Verification

• uv run pytest tests/test_agent_pi.py tests/test_e2e_user_agent.py::TestPiUserAgent — 42 passed
• uv run pytest tests/ -q (full unit suite, e2e excluded) — 943 passed
• uv run ruff check on changed files — clean
• Confirmed PI_CODING_AGENT_DIR is respected by inspecting the installed pi package source and by running pi --list-models with the env var set to a throwaway config dir (pi reported loading <dir>/models.json)