authn: remove the local agent-token plane

The agent_token table and everything that minted, verified, listed or revoked
it. Agent credentials are tokens.sr.ht working tokens now: signed, expiring,
owned by a meta.sr.ht account and carrying grants, verified locally by
sr-ht-ecore's bearer package. One door, and nothing behind it — a credential the
instance plane refuses is refused, where it used to be offered to a second store
that might say yes.

DEPLOY GATE: do not deploy this until every agent configured with the shared
secret holds a tokens.sr.ht token with spec:propose (spec:read to read).
Migration 0005 drops the table, migrate-on-upgrade runs it on deploy, and
deploying early locks out every agent at once — including the SSH push path.

hooks/ no longer reads agent_token directly. The push path goes through the same
authn.Resolver the HTTP surfaces use and demands spec:propose, because a push by
an agent is a proposal by another transport. The refs rule and the provenance
requirement are untouched on both counts: a universal grant is still only an
agent to the receive path, and X-Agent / X-Agent-Session are still mandatory on
every agent write.

bearer.ErrNotOurs is now a permanent refusal. A meta.sr.ht PAT used to fall
through to the local store and miss there; with no store to fall through to it
earns a 401 rather than the 503 an unclassified error would.

[tokens.sr.ht] origin becomes a required config key: with no issuer there is no
credential to check, so the daemon fails startup instead of serving reads and
refusing every agent write one request at a time.

/tokens redirects to the daemon that issues (tokens SPEC ch. 7).

feat: specsrht-migrate — brant wrapper for the spec.sr.ht schema

Single-service wrapper around git.sr.ht/~bitfehler/brant, a close sibling of
doltsrht-migrate: the brant subcommands (up, down, current, list, stamp,
validate, ping) plus an extra `init` that applies schema.sql wholesale and
stamps to head for a fresh install.

The DSN comes from [spec.sr.ht]connection-string unless --dsn overrides it;
a missing or empty value is a fatal error rather than a default connection.
Migrations load from ./migrations in a checkout, else from
<[sr.ht]assets>/migrations/spec.sr.ht. -a honours [spec.sr.ht]migrate-on-upgrade
and exits early when it is off. lib/pq's "postgres" driver replaces brant's
pgx default, which this module does not link.

Tests cover flag parsing, DSN precedence and the migrations-directory
resolution order without a database; the init/up round trips against a scratch
schema skip unless SPECSRHT_TEST_PG is set.