feat: add canvas create command

[heo001997]

Summary

Adds slackcli canvas create — the C in canvas CRUD. First of a 3-PR series (create → edit → delete) building on #31 (list + read).

Closes #62

What's added

• canvas create subcommand — --title, --content / --file / --stdin (mutually-exclusive markdown sources), --channel, --json, --workspace.
• SlackClient.createCanvas() → wraps canvases.create, JSON-wraps document_content as {type:"markdown", markdown}. No auth-type special-casing — routes through the existing request() dispatcher (works with standard + browser tokens).
• Types: CanvasCreateOptions, CanvasDocumentContent.
• README: create examples + token-support note — canvas create needs a Standard token (xoxb/xoxp) with canvases:write; browser tokens (xoxc/xoxd) can't do canvas CUD; list + read work with both.
• CHANGELOG entry.

Tests

• New src/commands/canvas.test.ts — unit tests for buildCreateParams (title-only, markdown→document_content, channel→channel_id, empty) and resolveCanvasMarkdown (mutual exclusion, inline content, missing-file error).
• bun test → 188 pass / 0 fail. bun run build → ok.
• Live-verified on a real workspace with a standard xoxp token: created a canvas tabbed into a channel; canvas_id returned.

Manual Tests

• Create canvas - Inline / From file / Stdin / Empty => OK!

Notes

• Part of a 3-PR canvas-CRUD series — suggested review/merge order:
  1. create (feat: add canvas create command #62) →
  2. edit (feat: add canvas edit and section lookup commands #63) →
  3. delete (feat: add canvas delete command #64).
• Code-independent; later branches rebase after each merge.
• bun run type-check surfaces one pre-existing error in src/lib/workspaces.ts (exists imported from fs/promises, a Bun runtime extension tsc doesn't model) — present on main, untouched by this PR. This PR adds zero new type errors.

[shahariaazam]

Thanks for the PR. I have requested few changes

[shahariaazam]

Major: tested function is not the one that runs

Why: buildCreateParams duplicates the param logic in SlackClient.createCanvas (slack-client.ts:461-470), but the create action only calls createCanvas. The 6 unit tests assert on buildCreateParams, so the production builder is untested and the suite stays green even if it breaks. Two copies will drift.

Fix: keep one builder (in the client), test that one, and remove buildCreateParams.

[shahariaazam]

Major: contradictory browser-token claim

Why: this table says canvas create with a browser token is not supported, but CHANGELOG.md:15 says it requires canvases:write "or browser auth" and the PR body says it works with both. The code routes through request() which dispatches to browserRequest, and only a standard token was live-tested, so the not-supported claim is unverified and inconsistent.

Fix: test canvases.create with a browser token, then make README and CHANGELOG agree.

[shahariaazam]

Minor: unused types

Why: CanvasCreateOptions and CanvasDocumentContent are never imported. The action's options is untyped and both builders inline JSON.stringify({type:'markdown', markdown}) instead of CanvasDocumentContent.

Fix: type the action param as CanvasCreateOptions and use CanvasDocumentContent, or remove the types.