Discord publishing (per-article threads, weekly summary threads,
discordChannelId field, weeklyThreads config) has been removed from
the skill. This functionality will be handled by separate skills
that depend on the Agent's message tooling.

Changes:
- Delete scripts/save-weekly-thread.js
- harvest.js: remove isoWeekKey/weekDateRange helpers, Discord manifest
  fields (discordChannelId, weekKey, weekRange, weeklyThreadId), and
  publishing comment block
- source-manage.js: remove CHANNEL column from list, discordChannelId
  from add/edit/usage
- config-template.json: remove weeklyThreads and discordChannelId fields
- source-management.md: remove Discord fields and Weekly Thread section
- SKILL.md: remove Publishing to Discord section, Workflow step 7,
  discordThreadId from registry format, Discord from description/intro
- README.md: remove all Discord references from capability table,
  architecture diagram, prerequisites, and limitations
- .gitignore: add .zcode/ workspace dir