test: add comprehensive e2e tests with dedicated test workspace

[natikgadzhi]

Batch 1.1: End-to-end tests

Set up a dedicated Slack test workspace and add real e2e tests for all commands.

Setup

• Create a test Slack workspace with known test data (channels, users, messages, files, canvases, emojis)
• Store xoxc and xoxd credentials as GitHub Actions secrets
• E2e tests run in CI with make e2e (existing pattern from saved_e2e_test.go, unread_e2e_test.go)

Commands to cover

• auth check
• message <url>
• message --channel --ts
• channels get
• channels list
• channels search
• channels members
• search
• saved
• unread (existing)
• users list
• users get
• users search
• files read
• reactions get
• emojis search
• canvases read

Test patterns

• Each command gets table + JSON output tests
• Verify exit codes, output structure, and key field presence
• Rate limit resilience (run tests sequentially with delays)
• Existing patterns in saved_e2e_test.go and unread_e2e_test.go

Depends on: #46, #47, #48, #49, #50, #51, #52