#13486: fix: allow Slack HTTP mode without app token

## Why\nSlack HTTP mode (Events API) does not require an app token, but the Slack channel plugin treated accounts as unconfigured unless both botToken + appToken were present. This prevents the provider from starting in HTTP mode, so the webhook handler is never registered and POST /slack/events falls through to the Control UI (405).\n\n## What changed\n- Treat Slack accounts as configured when:\n - mode="http": botToken + signingSecret\n - mode="socket" (default): botToken + appToken\n- Keep configured reporting consistent across status snapshots.\n\n## Result\nHTTP mode now starts and registers the webhook handler at channels.slack.webhookPath (default /slack/events) without requiring an app token. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates Slack channel configuration detection to support HTTP (Events API) mode without requiring an app-level token. It adds a shared `isSlackAccountConfigured()` helper and uses it consistently in `config.isConfigured`, `describeAccount`, and status snapshot building so configured/running reporting stays aligned. Main issue to address before merging: the interactive/CLI setup path still rejects configurations without `appToken` (`setup.validateInput`), so users can’t actually configure HTTP mode accounts even though they’re now treated as configured at runtime. <h3>Confidence Score: 4/5</h3> - Mostly safe to merge once the Slack HTTP-mode setup validation is aligned with the new configured logic. - The change is small and localized, and the new helper reduces inconsistency across configured/status reporting. The remaining blocker is a definite behavior mismatch: `setup.validateInput` still requires `appToken` even in HTTP mode, preventing users from configuring the newly-supported mode via the intended setup flow. - extensions/slack/src/channel.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->