Add slack-extended skill: file uploads, canvases, and bookmarks

[jilycn]

Summary

• Adds a community slack-extended skill that complements the core Slack skill
• File uploads via Slack's v2 upload API (files.getUploadURLExternal + files.completeUploadExternal)
• Canvas management — create, edit (append/replace/delete sections), delete, set access
• Bookmark management — add, list, edit, remove link bookmarks in channel bookmark bars

Details

• All scripts are Python 3 with zero external dependencies (stdlib urllib + json only)
• Bot token is read from ~/.openclaw/openclaw.json automatically
• Required Slack OAuth scopes: files:write, canvases:write, bookmarks:write, bookmarks:read
• Documents that Slack API does not support folder bookmarks (UI-only feature)

Test plan

• Tested slack_bookmark.py list against a live Slack channel — returns {"ok": true, "bookmarks": []}
• Tested slack_file_upload.py and slack_canvas.py in production OpenClaw deployment
• Reviewers can verify scripts against Slack API docs

🤖 Generated with Claude Code

[openclaw-barnacle]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fix invalid canvas access level in usage example

The documented access command uses --level edit, but scripts/slack_canvas.py only accepts --level values read or write (argparse choices), so users following this example hit an immediate CLI error and cannot apply canvas permissions until they discover the mismatch themselves.

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