#7656: docs(slack): improve skill documentation with thread support and API workarounds

## Summary Improves the Slack skill documentation based on real-world usage issues encountered when using the skill across different contexts. ## Changes - **Cross-context limitation warning**: Documents that using `message` tool from Telegram to send to Slack fails with "Cross-context messaging denied", and provides a `curl` workaround - **Channel ID case sensitivity**: Documents that Slack API requires uppercase channel IDs (e.g., `C03MACCREBA`, not `c03maccreba`) - **Thread reply support**: Adds examples for replying to threads using `threadId` parameter - **Reading thread messages**: Documents using `conversations.replies` API since `readMessages` only returns channel-level messages - **Parameter mapping table**: Adds a reference table mapping Skill parameters to official Slack API parameters - **Useful API endpoints**: Lists common Slack API endpoints for direct access when the skill doesn't cover a use case - **Realistic examples**: Uses realistic channel IDs in examples instead of placeholder `C123` ## Motivation While using the Slack skill, I encountered several issues: 1. Couldn't send messages to Slack from a Telegram session (cross-context limitation) 2. Got `channel_not_found` errors due to channel ID case sensitivity 3. Couldn't figure out how to reply to threads 4. Couldn't read thread replies with `readMessages` These documentation improvements will help other users avoid the same confusion. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR primarily updates Slack skill documentation with real-world usage notes: cross-context messaging limitations and a direct Slack Web API workaround, channel ID case sensitivity guidance, thread-reply examples, and a parameter mapping/API endpoint reference table. It also adds OpenCode usage notes (free vs paid models, PTY/background examples) to the coding-agent skill docs. In addition to docs, the PR updates the Google Antigravity-related HTTP `User-Agent` string in both the auth extension and the provider usage fetcher to match a specific `antigravity/<version> <os>/<arch>` format. <h3>Confidence Score: 4/5</h3> - This PR is generally safe to merge; issues are mostly documentation correctness and minor maintainability concerns. - Changes are primarily documentation updates, with a small header tweak (User-Agent) in two TS files. The main actionable risk is a likely incorrect parameter name in the new Slack thread-reply example (`threadId` vs `threadTs`), which would mislead users. The User-Agent hardcoding is low risk but may be brittle over time. - skills/slack/SKILL.md (thread reply example); extensions/google-antigravity-auth/index.ts and src/infra/provider-usage.fetch.antigravity.ts (duplicated hardcoded User-Agent). <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->