feat(message): Slack-native drafts — message drafts list/create/update/delete

[mikaelq]

Implements #83 (option 1: separate plural drafts subcommand, keeping the existing message draft browser editor unchanged).

What

New message drafts subcommand group backed by Slack's undocumented drafts.* session endpoints (the same API the official clients use), so agents can read and write drafts that show up natively in the user's Slack client:

agent-slack message drafts list [--limit <n>] [--all]
agent-slack message drafts create <target> <text> [--thread-ts <ts>] [--broadcast]
agent-slack message drafts update <draft-id> <text> [--channel <target>] [--thread-ts <ts>] [--broadcast] [--last-updated-ts <ts>]
agent-slack message drafts delete <draft-id> [--last-updated-ts <ts>]

• create accepts the usual targets: #channel/name, channel id, user id (opens a DM), or a message URL (drafts a thread reply, same semantics as message send).
• Draft text goes through the existing mrkdwn → rich_text conversion (textToRichTextBlocks), so lists/bold/code/mentions render natively.
• update replaces the body but preserves the draft's destination and attached files unless overridden; the conflict-detection client_last_updated_ts is auto-fetched for update/delete when omitted (including Slack's 7-digit fractional-padding quirk).
• list resolves channel/DM display names best-effort.
• Requires browser-style auth (xoxc/xoxd), like the other session-endpoint features (later, unreads); noted in the docs.

Structure

Follows the message scheduled pattern: src/slack/drafts.ts (API layer) + src/cli/message-drafts-actions.ts / message-drafts-command.ts (CLI layer). Docs updated in README, skills/agent-slack/SKILL.md, and references/commands.md.

Testing

• 16 unit tests in test/drafts.test.ts (wire payloads, ts padding, parsing, auto-fetch/error paths); full suite, typecheck, lint, and format all pass.
• Verified live against a real workspace: create → appears in the Slack client's Drafts → list (with resolved names) → update → delete, including thread-reply drafts via message-URL targets.

Closes #83

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