Add channel mark command

[robert10997]

Summary

• Adds channel mark subcommand wrapping Slack's conversations.mark API
• Marks channels, DMs, and group DMs as read up to a given message timestamp
• Accepts message URLs (extracts channel + ts automatically), channel names/IDs, or DM channel IDs
• Updates SKILL.md, commands.md, targets.md, and output.md reference docs

Usage

# By URL
agent-slack channel mark "https://workspace.slack.com/archives/C123/p1700000000000000"

# By channel name + ts
agent-slack channel mark "#general" --workspace "myteam" --ts "1770165109.628379"

# By DM channel ID + ts
agent-slack channel mark "D0A04PB2QBW" --workspace "myteam" --ts "1770165109.628379"

Test plan

• bun run typecheck passes
• bun run lint passes (0 warnings, 0 errors)
• bun run test passes (125 tests)
• bun run format:check passes
• Manually tested against live workspace: marking channels, DMs, and Slack Connect DMs as read

[AmethystLiang]

Here is the review from Codex:

channel mark is a worthwhile addition and fits the CLI well overall, but there is one blocking issue before merge. When <target> is a Slack message URL, the workspace should come from the URL and must not be overridden by --workspace. Right now the implementation lets --workspace take precedence in URL mode, which is inconsistent with the existing URL-based message commands and creates a real footgun for a mutating command: a permalink from workspace A can be executed against workspace B. Please make URL targets authoritative here (or reject --workspace entirely when a URL target is passed), and add a command-level test covering that behavior along with the --ts validation path.

Once addressed, I will merge it.

[robert10997]

Thank you, updated!

[AmethystLiang]

Looks good! Clean implementation that follows existing patterns well. Added a commit to allow --ts override with URL targets (useful for the mark-as-unread pattern) while keeping --workspace rejection.