Add canvas creation from Markdown

[nwparker]

Summary

Adds agent-slack canvas create, giving agents a first-class way to create Slack canvases from either a local Markdown file or an inline Markdown blob.

• accepts exactly one of --file <path> or --markdown <text>
• supports an optional --title
• supports --channel <id-or-name> to add the canvas as a channel tab (and cover Slack free plans, where channel_id is required)
• uses the existing workspace selection, channel resolution, and auth auto-refresh paths
• returns compact JSON as canvas: { id, title?, channel_id? }
• rejects empty content and malformed success responses before presenting a false success

Authentication paths

Live testing found that Slack's public canvases.create API rejects imported xoxc browser credentials with not_allowed_token_type. Canvas creation now selects the appropriate API automatically:

• standard Slack tokens use the official canvases.create API with Markdown document_content
• imported browser credentials use Slack's multipart files.createCanvas Markdown path
• --channel remains standard-token-only because the browser-auth path creates standalone canvases and cannot add a channel tab

Standard tokens require Slack's canvases:write scope.

Usage

# Create from a file
agent-slack canvas create --file ./launch-plan.md --title "Launch plan"

# Create from an inline Markdown blob
agent-slack canvas create --markdown $'# Launch plan\n\n- [ ] Ship it' --title "Launch plan"

# Add the canvas as a channel tab (required on free Slack plans; standard tokens only)
agent-slack canvas create --file ./launch-plan.md --channel "project-launch"

When multiple workspaces are configured, callers can use the standard --workspace <url-or-unique-substring> selector. Channel names go through the same ambiguity checks and name-to-ID resolution used by the rest of the CLI.

Validation and failure behavior

• missing --file/--markdown is rejected before authentication
• supplying both source options is rejected before authentication
• whitespace-only Markdown is rejected
• file read failures include the source path and underlying error
• the Markdown bytes read from a file or supplied inline are preserved in the API request
• a Slack success response without a canvas/file ID is treated as an error
• channel-tab creation with imported browser credentials fails locally with an actionable standard-token requirement

Documentation

Kept all shipped agent guidance in sync with the implementation:

• updated the README feature list, command map, usage examples, output shape, auth requirements, and free-plan note
• updated skills/agent-slack/SKILL.md common commands and mutation safety guidance
• updated the bundled command and output references
• updated llms.txt feature metadata

Testing

Live acceptance testing in stablygroup passed for both supported sources using imported browser credentials:

• created a canvas from a Markdown file and read it back successfully
• created a canvas from an inline Markdown blob and read it back successfully
• sent both canvas links to the requested self-DM and verified delivery

Automated verification:

• bun test — 265 passed, 0 failed
• focused canvas/client regression suite — 14 passed, 0 failed
• bun run typecheck
• bun run build
• bun run build:npm
• bun run lint — 0 errors (10 existing warnings in unrelated files)
• bun run format:check
• git diff --check