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

CHANGELOG.md

@@ -0,0 +1,36 @@
+# Changelog
+
+All notable changes to this project are documented here.
+The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
+
+## [1.0.0] - 2026-10-19
+
+First public release of the ESP32-C3 CO₂ & environment sensor board (prototype v1).
+
+### Added
+
+- **Hardware design** (KiCad 10): schematic, 2-layer board layout, custom symbol
+  library (`esp_sensair_stuff.kicad_sym`) and footprint library
+  (`esp_sensair_stuff.pretty/`) for the ESP32-C3 module, Sensirion S8 and BME280.
+- Project-local library tables (`fp-lib-table`, `sym-lib-table`) so the design
+  opens with all custom parts resolved on a fresh clone.
+- **Fabrication data** in `gerber/`: Gerber X2 plot, Excellon PTH/NPTH drills,
+  Gerber job/stackup file, and a ready-to-upload `esp-sensair.zip`.
+- **Bill of materials** (`bom/esp-sensair-bom.csv`) plus the human-readable BOM
+  in the README.
+- **Documentation** in `docs/`: schematic PDF, top/bottom layer plots, 3D render.
+- **Firmware** (`firmware/`): ESPHome configuration and secrets templates
+  (`*.example`) for the ESP32-C3 (esp-idf), BME280 over I²C and Sensirion S8
+  over UART, with Home Assistant API (noise encryption) and OTA.
+- **Licensing**: CERN-OHL-S-2.0 for hardware (`LICENSE`), MIT for firmware, BOM
+  and docs (`LICENSE.MIT`).
+- `.gitignore` covering KiCad local history/backups/lock files, ESPHome build
+  artifacts, live firmware credentials, and OS/editor noise.
+
+### Known issues
+
+See the "Known issues and lessons learned (v1 → v2)" section of the
+[README](README.md). Highlights: power delivered through the module's USB-C 5 V
+path, single decoupling capacitor, no mounting holes, no populated I²C pull-ups,
+unannotated schematic, leaded HASL, and a `+` in the project name that leaks
+into Gerber file names.