Replace the hardcoded `join(homedir(), ".pi", "agent", ...)` defaults
with pi's `getAgentDir()` helper so config and session paths honour
`PI_CODING_AGENT_DIR` relocation.

- src/core/settings.ts: derive pi-vcc-config.json default from getAgentDir()
  (PI_VCC_CONFIG_PATH override preserved)
- tests/support/real-sessions.ts: resolve ~/.pi/agent/sessions via getAgentDir()