docs: document the CI pipeline (task order nova-platform#3)

The workflow landed in PR #6 but nothing in the repo pointed a reader at it.
This states what the three legs gate, what a loud failure means, and the
local equivalent. No product behaviour changes.

run: hourly RUN 2 2026-10-26T02:0xZ

docs/ci.md

@@ -0,0 +1,26 @@
+# Continuous integration - nova-platform
+
+**TL;DR -** Every push and every pull request to `main` runs the CI
+workflow (`.gitea/workflows/ci.yml`). It is the offline mirror of
+`scripts/run_ci.sh`: lint, test, check-only. A leg that cannot run fails
+loud - nothing skipped is reported as passed.
+
+**What** -
+
+- **lint** - `python3 -m py_compile` over `core/`, `adapters/`, `scripts/`
+  (stdlib only, always runs).
+- **test** - `pytest tests/ -q -m "not shell"`, with the test dependencies
+  from `requirements-test.txt` installed first when pypi is reachable; a
+  missing toolchain fails the leg with its absence list.
+- **check-only** - `bash scripts/run_platform.sh --check-only
+  contracts/static-assets.yml` (needs `yaml` + `jsonschema`).
+
+**Why** - the line shipped without a pipeline, so every change went out
+ungated. The workflow runs the same three legs as the local script, so a
+green local run and a green pipeline mean the same thing.
+
+**Action** - none - FYI. The pipeline was ordered under
+`coreci/nova-platform#3`; the workflow itself landed in PR #6.
+
+**Details** - local equivalent: `bash scripts/run_ci.sh`. Run history:
+the repository's Actions tab.