Make repo release-ready: add sym-lib-table, CHANGELOG, expand .gitignore

- Add project-local sym-lib-table so the custom esp_sensair_stuff symbols
  resolve on a fresh clone (fp-lib-table already covered footprints)
- Add CHANGELOG.md documenting the v1.0.0 release
- Expand .gitignore: KiCad lock files, ESPHome build artifacts, Python
  cache, editor/OS noise, logs, merge artifacts
- README: list sym-lib-table and CHANGELOG.md in the repository layout

sym-lib-table

@@ -0,0 +1,4 @@
+(sym_lib_table
+	(version 7)
+	(lib (name "esp_sensair_stuff") (type "KiCad") (uri "${KIPRJMOD}/esp_sensair_stuff.kicad_sym") (options "") (descr ""))
+)