fix(gateway): honor MATRIX_HOME_ROOM in onboarding

[mikeyobrien]

Bug Description

Matrix home delivery is configured with MATRIX_HOME_ROOM, but Hermes could still warn that no Matrix home target was set. The warning path and /sethome were not using the same home-target env-var mapping, so Matrix looked misconfigured even when it was correct.

Fixes N/A

Root Cause

gateway/run.py still had logic that assumed MATRIX_HOME_CHANNEL in the onboarding warning path, while Matrix actually uses MATRIX_HOME_ROOM. /sethome and onboarding were not grounded on the same shared mapping.

Fix

• use the shared home-target env-var mapping in gateway onboarding warnings
• make /sethome use the same mapping instead of assuming *_HOME_CHANNEL
• add regression coverage for Matrix and fallback behavior

How to Verify

1. Configure Matrix home delivery with MATRIX_HOME_ROOM.
2. Start the gateway and confirm Hermes does not emit the false "No home channel is set for Matrix" warning.
3. Run /sethome for Matrix and confirm the command resolves the same env-var mapping as onboarding.
4. Run the regression tests below.

Test Plan

• Added regression test for this bug
• Existing tests still pass
• Manual verification of the fix

Commands run:

• PYTEST_ADDOPTS= pytest -q tests/gateway/test_home_target_env_var.py tests/gateway/test_steer_command.py tests/gateway/test_unauthorized_dm_behavior.py
• PYTEST_ADDOPTS= pytest -q tests/gateway/test_home_target_env_var.py tests/gateway/test_matrix.py tests/cron/test_scheduler.py

Risk Assessment

Low — the change narrows behavior onto the existing shared home-target mapping and adds regression coverage around Matrix and fallback resolution.

[teknium1]

Thanks @mikeyobrien — merged as PR #18272. Your fix was the right approach: single source of truth via _HOME_TARGET_ENV_VARS so /sethome and onboarding agree on the env-var name. Your commit was cherry-picked with authorship preserved (commit 1be3b74 on main), and the regression test was widened to also cover email (EMAIL_HOME_ADDRESS) and the convention platforms.

#18272