slck: canvas support (create, edit, channel canvases)

[rianjs]

Feature Request

There's no way to create or manage Slack Canvases via slck. Canvases are Slack's native rich-document format and accept markdown content directly — making them ideal for posting formatted reports, runbooks, and summaries from the CLI.

Current Behavior

No canvas command exists. The closest alternative is Block Kit (--blocks), which requires JSON and uses Slack's mrkdwn flavor rather than standard markdown.

Proposed Solution

Add a slck canvas command group:

# Create a standalone canvas
slck canvas create --title "Sprint Review" --file report.md
slck canvas create --title "Sprint Review" --text "# Heading\n\nContent here"
echo "# Report" | slck canvas create --title "Report" --stdin

# Create a channel canvas (pinned to channel tab)
slck canvas create --channel C1234567890 --title "Runbook" --file runbook.md

# Edit an existing canvas
slck canvas edit <canvas-id> --file updated.md

# Delete a canvas
slck canvas delete <canvas-id>

The --file flag should read a markdown file and pass it as the document_content payload.

API Reference

Slack's Canvas API accepts markdown natively via the document_content object:

{
  "type": "markdown",
  "markdown": "# Heading\n\n**Bold** and *italic* content..."
}

Supported formatting: headings (h1-h3), bold, italic, strikethrough, bulleted/ordered lists, checklists, code blocks/spans, markdown tables, dividers, links, @mentions, emoji.

Operation	Endpoint
Create standalone canvas	canvases.create
Create channel canvas	conversations.canvases.create
Edit canvas	canvases.edit
Delete canvas	canvases.delete

Docs:

• https://docs.slack.dev/reference/methods/canvases.create/
• https://docs.slack.dev/surfaces/canvases/

Version

slck version 0.1.0