Support DATASETTE_URL and DATASETTE_TOKEN environment variables

Allow configuring a base Datasette instance URL and API token via
environment variables, so users can run commands like
`dclient query data "select ..."` without specifying a full URL each time.

DATASETTE_URL is combined with the argument as a path segment.
DATASETTE_TOKEN is used as a lowest-priority fallback for auth.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/environment.md

@@ -0,0 +1,68 @@
+(environment-variables)=
+
+# Environment variables
+
+`dclient` supports two environment variables for convenient access to a Datasette instance without needing aliases or repeated URLs.
+
+## DATASETTE_URL
+
+Set this to the base URL of your Datasette instance:
+
+```bash
+export DATASETTE_URL=https://my-instance.datasette.cloud
+```
+
+Then pass just the database name as the first argument to any command:
+
+```bash
+dclient query data "select * from my_table limit 10"
+```
+
+This is equivalent to:
+
+```bash
+dclient query https://my-instance.datasette.cloud/data "select * from my_table limit 10"
+```
+
+It works with all commands:
+
+```bash
+dclient insert data my_table data.csv --csv
+dclient actor data
+```
+
+Full URLs and aliases always take priority over `DATASETTE_URL`. If the argument starts with `http://` or `https://`, it is used as-is. If it matches an alias in `aliases.json`, the alias is used.
+
+## DATASETTE_TOKEN
+
+Set this to an API token:
+
+```bash
+export DATASETTE_TOKEN=dstok_abc123
+```
+
+The token will be used automatically for any request that doesn't have a more specific token configured.
+
+The precedence order for tokens is:
+
+1. `--token` CLI flag (highest priority)
+2. Stored token from `auth.json` (matched by URL prefix)
+3. `DATASETTE_TOKEN` environment variable (lowest priority)
+
+## Using both together
+
+These variables work well together for quick access to a single instance:
+
+```bash
+export DATASETTE_URL=https://my-instance.datasette.cloud
+export DATASETTE_TOKEN=dstok_abc123
+
+# Query the "data" database
+dclient query data "select * from my_table"
+
+# Insert into the "data" database
+cat records.json | dclient insert data my_table - --json
+
+# Check your actor identity
+dclient actor data
+```