docs: add contributor coding standards

docs/coding-standards.md

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+# Coding Standards
+
+These standards apply to all contributions and complement [CONTRIBUTING.md](../CONTRIBUTING.md).
+
+## Go
+
+- Run `gofmt` (or `goimports`) before committing; CI does not auto-format.
+- Package comments start with `// Package <name> ...` and describe purpose, not implementation.
+- Exported identifiers must have a doc comment starting with their name.
+- Wrap errors with context: `fmt.Errorf("package: doing thing: %w", err)`.
+- Prefer table-driven tests; name test files `<file>_test.go` alongside the code they test.
+
+## Dashboard (TypeScript)
+
+- Run `pnpm typecheck` in `web/dashboard` before opening a PR.
+- Keep components small and colocate types with the component that owns them.
+
+## Commits and PRs
+
+- One logical change per PR; keep new files focused and reasonably small.
+- Use Conventional Commits (see CONTRIBUTING.md) and reference the issue with `closes #<number>`.
+- Do not commit secrets or `.env` files; use `.env.example` as the template.
+
+## Review Expectations
+
+- All CI checks (`go build`, `go test`, dashboard typecheck) must pass before merge.
+- Address review feedback with follow-up commits rather than force-pushing, unless asked to squash.