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>

CHANGELOG.md

@@ -0,0 +1,63 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [2.1.0] - 2026-02-15
+
+### Highlights
+
+Three headline features: watchlists for always-on bots, YouTube transcripts as a 4th source, and Codex CLI compatibility. Plus bundled X search with no external CLI needed.
+
+### Added
+
+- Open-class skill with watchlists, briefings, and history modes (SQLite-backed, FTS5 full-text search, WAL mode) (`feat(open)`)
+- YouTube as a 4th research source via yt-dlp -- search, view counts, and auto-generated transcript extraction (`feat: Add YouTube`)
+- OpenAI Codex CLI compatibility -- install to `~/.agents/skills/last30days`, invoke with `$last30days` (`feat: Add Codex CLI`)
+- Bundled X search -- vendored subset of Bird's Twitter GraphQL client (MIT, originally by @steipete), no external CLI needed (`v2.1: Bundle Bird X search`)
+- Native web search backends: Parallel AI, Brave Search, OpenRouter/Perplexity Sonar Pro (`feat(engine)`)
+- `--diagnose` flag for checking available sources and authentication status
+- `--store` flag for SQLite accumulation (open variant)
+- Conversational first-run experience (NUX) with dynamic source status (`feat(nux)`)
+
+### Changed
+
+- Smarter query construction -- strips noise words, auto-retries with shorter queries when X returns 0 results
+- Two-phase search architecture -- Phase 1 discovers entities (@handles, r/subreddits), Phase 2 drills into them
+- Reddit JSON enrichment -- real upvotes, comments, and upvote ratio from reddit.com/.json endpoint
+- Engagement-weighted scoring: relevance 45%, recency 25%, engagement 30% (log1p dampening)
+- Model auto-selection with 7-day cache and fallback chain (gpt-4.1 -> gpt-4o -> gpt-4o-mini)
+- `--days=N` configurable lookback flag (thanks @jonthebeef, [#18](https://github.com/mvanhorn/last30days-skill/pull/18))
+- Model fallback for unverified orgs (thanks @levineam, [#16](https://github.com/mvanhorn/last30days-skill/pull/16))
+- Marketplace plugin support via `.claude-plugin/plugin.json` (inspired by @galligan, [#1](https://github.com/mvanhorn/last30days-skill/pull/1))
+
+### Fixed
+
+- YouTube timeout increased to 90s, Reddit 429 rate limit fail-fast
+- YouTube soft date filter -- keeps evergreen content instead of filtering to 0 results
+- Eager import crash in `__init__.py` that broke Codex environments
+- Reddit future timeout (same pattern as YouTube timeout bug)
+- Process cleanup on timeout/kill -- tracks child PIDs for clean shutdown
+- Windows Unicode fix for cp1252 emoji crash (thanks @JosephOIbrahim, [#17](https://github.com/mvanhorn/last30days-skill/pull/17))
+- X search returning 0 results on popular topics due to over-specific queries
+
+### New Contributors
+
+- @JosephOIbrahim -- Windows Unicode fix ([#17](https://github.com/mvanhorn/last30days-skill/pull/17))
+- @levineam -- Model fallback for unverified orgs ([#16](https://github.com/mvanhorn/last30days-skill/pull/16))
+- @jonthebeef -- `--days=N` configurable lookback ([#18](https://github.com/mvanhorn/last30days-skill/pull/18))
+
+### Credits
+
+- @steipete -- Bird CLI (vendored X search) and yt-dlp/summarize inspiration for YouTube transcripts
+- @galligan -- Marketplace plugin inspiration
+- @hutchins -- Pushed for YouTube feature
+
+## [1.0.0] - 2026-01-15
+
+Initial public release. Reddit + X search via OpenAI Responses API and xAI API.
+
+[2.1.0]: https://github.com/mvanhorn/last30days-skill/compare/v1.0.0...v2.1.0
+[1.0.0]: https://github.com/mvanhorn/last30days-skill/releases/tag/v1.0.0