#702 serve-auth: SWAMP_SERVE_URL env var as default for --server flag

Problem

Every remote command requires `--server wss://...` on every invocation:

```bash swamp access grant list --server wss://demo.swamp-club.ai swamp access can-i --server wss://demo.swamp-club.ai swamp access reload --server wss://demo.swamp-club.ai swamp model method run hello execute --input 'run=echo hi' --server wss://demo.swamp-club.ai ```

This is tedious when working against a single server for an extended session.

Solution

Add `SWAMP_SERVE_URL` as an env var default for the `--server` flag on all commands that support it. Follows the naming convention of `SWAMP_SERVE_CERT_FILE` and `SWAMP_SERVE_KEY_FILE`.

```bash export SWAMP_SERVE_URL=wss://demo.swamp-club.ai swamp access grant list swamp access can-i swamp access reload swamp model method run hello execute --input 'run=echo hi' ```

`--server` flag takes precedence over the env var when both are provided.

Commands to update

All commands with a `--server` flag:

• `swamp model method run`
• `swamp workflow run`
• `swamp access grant create/list/revoke`
• `swamp access group create/add-member/remove-member/list/members`
• `swamp access check`
• `swamp access reload`
• `swamp access can-i`

Priority

Low — UX improvement, not blocking anything.