docs: add CHANGELOG.md, release notes, and v2.1 polish

Add CHANGELOG.md (Keep a Changelog format) and release-notes.md
for the v2.1.0 GitHub Release. Also includes queued copy updates:
plugin.json version bump to 2.1.0, YouTube added to descriptions,
SKILL.md security/permissions section and ClawdBot metadata,
updated watchlist examples, and v2.1 launch post refinements.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lib/ui.py

@@ -108,10 +108,9 @@ You can unlock more sources with API keys — just ask me how and I'll walk you
 Some examples of what you can do:
 - "last30 what are people saying about Figma"
 - "last30 watch my biggest competitor every week"
-- "last30 tell me once a month about what my board members are up to"
-- "last30 give me my morning briefing"
+- "last30 watch Peter Steinberger every 30 days"
+- "last30 watch AI video tools monthly"
 - "last30 what have you found about AI video?"
-- "last30 set a budget so you don't go crazy"
 
 Just start with "last30" and talk to me like normal.
 """