refactor: Avoid conflation of Space and Account (#1545)

* `space.signer.withDID(account)` gives us a Signer with the Space's
key, but which reports the Account's DID. It looks like this might have
been needed at some point when this thing was actually used to sign
something. But as of now, we don't need a *Signer*, just a DID, and a
`did:mailto:` is fine. So, simplify this.

* This was being passed as the `agent`, but it's not an Agent.
`createAuthorization()` doesn't need an Agent, it needs an *audience*.
In some cases that's an Agent, in some cases it's an Account. This
changes the name to clarify.

packages/access-client/src/space.js

@@ -1,5 +1,5 @@
 import * as ED25519 from '@ucanto/principal/ed25519'
-import { delegate, Schema, UCAN, error, fail } from '@ucanto/core'
+import { delegate, Schema, UCAN, error, fail, DID } from '@ucanto/core'
 import * as BIP39 from '@scure/bip39'
 import { wordlist } from '@scure/bip39/wordlists/english'
 import * as API from './types.js'
@@ -67,7 +67,7 @@ export const toMnemonic = ({ signer }) => {
  */
 export const createRecovery = (space, account) =>
   createAuthorization(space, {
-    agent: space.signer.withDID(account),
+    audience: DID.parse(account),
     access: Access.accountAccess,
     expiration: Infinity,
   })
@@ -85,21 +85,21 @@ export const SESSION_LIFETIME = 60 * 60 * 24 * 365
  *
  * @param {Model} space
  * @param {object} options
- * @param {API.Principal} options.agent
+ * @param {API.Principal} options.audience
  * @param {API.Access} [options.access]
  * @param {API.UTCUnixTimestamp} [options.expiration]
  */
 export const createAuthorization = async (
   { signer, name },
   {
-    agent,
+    audience,
     access = Access.spaceAccess,
     expiration = UCAN.now() + SESSION_LIFETIME,
   }
 ) => {
   return await delegate({
     issuer: signer,
-    audience: agent,
+    audience: audience,
     capabilities: toCapabilities({
       [signer.did()]: access,
     }),
@@ -174,7 +174,7 @@ export class OwnedSpace {
       return fail('Please provide an agent to save the space into')
     }
 
-    const proof = await createAuthorization(this, { agent })
+    const proof = await createAuthorization(this, { audience: agent })
     await agent.importSpaceFromDelegation(proof)
     await agent.setCurrentSpace(this.did())
 
@@ -210,13 +210,13 @@ export class OwnedSpace {
    * specified ability (passed as `access.can` field) on the this space.
    * Optionally, you can specify `access.expiration` field to set the
    *
-   * @param {API.Principal} agent
+   * @param {API.Principal} principal
    * @param {object} [input]
    * @param {API.Access} [input.access]
    * @param {API.UCAN.UTCUnixTimestamp} [input.expiration]
    */
-  createAuthorization(agent, input) {
-    return createAuthorization(this, { ...input, agent })
+  createAuthorization(principal, input) {
+    return createAuthorization(this, { ...input, audience: principal })
   }
 
   /**