feat: Discord quickstart UX — plugin check, general channel selection, options output

[changhansung]

Problem

Discord users face incomplete setup UX in agend quickstart:

1. No plugin check — Users select Discord but discover @suzuke/agend-plugin-discord is missing only at runtime
2. No general channel selection — options.general_channel_id is never asked, so general dispatcher / approval prompts don't work after setup
3. No category name option — Users must manually edit fleet.yaml to set options.category_name

Fix

All changes in src/quickstart.ts (+73/-6):

Plugin Check

• Before Discord token input, checks npm list -g @suzuke/agend-plugin-discord --depth=0
• If not found: warns with install command, asks "Continue anyway? [Y/n]"
• Non-blocking — user can proceed without plugin

General Channel Selection (new Step 5/5)

• Calls Discord API GET /guilds/{guild_id}/channels to list text channels
• Filters type === 0 (GuildText), sorts by name, limits to 20
• User selects from numbered list; fallback to manual ID input if API fails
• NaN input guard: isNaN(parsed) ? 0 : clamp(parsed)

Category Name

• Asks category name with default "AgEnD Agents"
• Only written to fleet.yaml when non-default

fleet.yaml Output

channel:
  type: discord
  ...
  options:
    general_channel_id: "345678901"
    category_name: "Custom Name"  # only if non-default

Step Numbering

• Discord: 5 steps (Backend → Channel → Bot → Guild & User → General Channel)
• Telegram: unchanged at 4 steps

Tests

tests/quickstart.test.ts — 14 new test cases:

• listDiscordChannels: API success, API error, network failure
• Channel filtering: type 0 only, sort + limit 20
• Input validation: NaN default, empty default, out-of-range clamp
• Options output: general_channel_id, category_name non-default, default omission, empty ID
• Full fleet.yaml structure with options block
• Plugin check command pattern

All 505 tests pass (60 test files).