feat(web,service): the owner mints and revokes agent tokens in a browser

Issuing a credential required SSH to the host, which made the remote
agent write plane unusable from anywhere else: to hand an agent a token
the owner had to be at the machine. /tokens is that page — list, mint,
revoke — behind the same owner-only gate and same-origin guard as
approve/reject.

The mint is owner-only, and that rule is what revocation depends on: an
agent allowed to mint would survive having its own credential revoked by
issuing itself another, and "revoke the token" is the entire incident
response this design has. An agent asking for the page gets 403 rather
than the read plane's login redirect — it is authenticated already, so
bouncing it to meta would answer a question it did not ask.

The plaintext is rendered in the response to the POST rather than after a
redirect. A redirect would either drop the secret or carry it in a URL,
where it lands in history and in every proxy log on the way; the cost is
that a reload re-submits and mints a second token, which is one click to
revoke on that same page, whereas a lost token is not recoverable.

service.IssueAgentToken/ListAgentTokens/RevokeAgentToken hold the ACL and
the mint, and `specsrht token` now goes through them too, so the CLI and
the page cannot drift into two ideas of what issuing a token is.

spec-ejq.3