docs: add v0.20.0 release notes with auto-update feature

Signed-off-by: Drew Cain <groksrc@gmail.com>

Author: groksrc

content/2.whats-new/0.v0.19.0.md content/2.whats-new/0.v0.20.0.mdcontent/2.whats-new/0.v0.19.0.md renamed to content/2.whats-new/0.v0.20.0.md

@@ -1,9 +1,43 @@
 ---
-title: v0.19.0
-description: Schema system, semantic search, per-project routing, and more in Basic Memory v0.19.0.
+title: v0.20.0
+description: Auto-update system, schema system, semantic search, per-project routing, and more in Basic Memory v0.19–v0.20.
 ---
 
-Basic Memory is a knowledge base that you and your AI share — notes you write in one conversation are available in the next. v0.19.0 is a major release that makes your notes more consistent, your searches smarter, and your projects more flexible.
+## Auto-Update (v0.20.0)
+
+Basic Memory now keeps itself up to date. CLI installs via Homebrew or uv tool automatically check for new versions in the background — no action required.
+
+The updater detects how you installed Basic Memory and uses the right update method:
+
+- **Homebrew** — checks `brew outdated` and runs `brew upgrade basic-memory`
+- **uv tool** — checks PyPI for the latest version and runs `uv tool upgrade basic-memory`
+- **uvx** — skipped (uvx always fetches the latest on each run)
+
+Update checks happen once per day by default and never block your workflow. The check runs in a background thread before the MCP server starts, so there's no impact on tool responsiveness.
+
+For manual control:
+
+```bash
+# Check and apply updates now
+bm update
+
+# Check only, don't install
+bm update --check
+```
+
+Three config fields control the behavior:
+
+- `auto_update` — enable or disable automatic checks (default: `true`)
+- `update_check_interval` — seconds between checks (default: `86400` / 24 hours)
+- `auto_update_last_checked_at` — tracks the last check time automatically
+
+See [Configuration](/reference/configuration) for details.
+
+---
+
+# v0.19.0
+
+v0.19.0 is a major release that makes your notes more consistent, your searches smarter, and your projects more flexible.
 
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