fix(gateway): respect explicit plugin platform disables

[ben-z]

Summary

• Preserve explicit platform enabled state for all YAML platform configs, not only Slack
• Skip plugin auto-enable when a platform was explicitly disabled
• Add a Discord regression test covering discord.enabled: false with YAML env bridging

Test Plan

• /Users/ben/.hermes/hermes-agent/venv/bin/python -m pytest tests/gateway/test_config.py -q -o 'addopts='
• git diff --check

[alt-glitch]

Duplicate of #31596 — same fix target (_apply_env_overrides() ignoring explicit enabled: false for plugin platforms). #31596 is already triaged and linked to the root cause family (#30736, #31109, #30803). Also competing with #18418 (Telegram variant) and #31468 (Feishu variant).

[Morad37]

Clean fix. The marker was being consumed by without checking whether the user had explicitly set in config. The extra guard preserves the user's explicit disable while still letting env-var auto-enable work for platforms that weren't configured at all.

The test covers the exact regression case -- config has but no token env var, so auto-enable would have overridden it before this fix.

[teknium1]

Thanks for the focused gateway regression fix. This automated hermes-sweeper review found that its exact guarantee is already implemented on current main.

• gateway/config.py:1255-1266 records _enabled_explicit for every platform configuration that explicitly supplies enabled, extending beyond the Slack-only behavior in this PR's base.
• gateway/config.py:2148-2161 skips registry-driven plugin auto-enablement when the platform is explicitly disabled, including the token/SDK-present path described in the PR.
• Current regression coverage in tests/gateway/test_slack_mention.py:515-563 verifies explicit disable wins over an environment token for both top-level and nested platform YAML.
• Commit 5600105478ffde29d7566b45421b100eaa29c4ef carried this behavior through the bundled-platform migration; it shipped in v2026.7.1.

This also matches the duplicate relationship noted in the discussion with #31596.