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: projects — a saved filter over one global index, not a container

A project is a named space set; querying one filters the single global
index. The meta-project is an implicit filter at a reserved address rather
than a row: a stored +everything would need a sync job on every space
creation, and its one failure mode is silently omitting a space.

SpaceFilter distinguishes All from an empty member list, because a freshly
created project has no members and must mean selects-nothing. Collapsing
the two would make every new project silently match the whole corpus.

refactor: move the reconciler's two deletes down to the layers that own them

The reconciler reached past its layer twice, because the primitives it
needed did not exist: a raw DELETE FROM proposal — the only SQL written
outside db/ — and a go-git RemoveReference under gitx's write lock. Both
move down, with no change in behaviour.

db.Store.DeleteOpenProposal keeps the guard in the statement, as
resolveProposal does, and distinguishes the two ways it can bite: a row
that is gone is ErrNotFound, a row that has been resolved is the new
ErrProposalNotOpen, which tells the reconciler "nothing to repair" apart
from "the repair no longer applies".

gitx.Repo.DeleteProposalBranch refuses anything outside proposals/* —
the only thing between a caller bug and a deleted approved branch — takes
the per-space write lock like every other write, and treats an
already-absent branch as success: the repair is a postcondition, and the
ref may legitimately vanish between the listing that found it and the
delete.

feat: Postgres schema, initial migration and the db/ query layer

The six tables of the design's Postgres schema — space, document_id,
proposal, agent_token, index_stamp, digest_mark — plus a typed query layer
over them. project and comment stay deliberately absent.

Two invariants are enforced in SQL rather than in Go, because a
check-then-write in Go has a window and this does not:

  - Global document ID uniqueness is the doc_id PRIMARY KEY, and the merge
    path's multi-row upsert guards its ON CONFLICT branch on the owning
    space, so a batch claiming another space's ID leaves that row untouched
    and surfaces as a CollisionError naming where the ID really lives.
  - Proposal transitions run as UPDATE ... WHERE state = 'open'; the current
    state is read back only to name it in the error. CHECK constraints make
    an unknown state, a merged row without an approval kind or merged_rev,
    and empty provenance unwritable by anything, including SQL that does not
    go through this package.

MergeProposal wraps the transition and the registry re-pointing in one
transaction: a merged proposal whose documents are still registered at their
pre-merge paths would break link resolution and the next staleness check.

Agent tokens store only a SHA-256 hash; the lookup boundary re-verifies with
subtle.ConstantTimeCompare and rejects revoked tokens distinctly.

Tests skip when SPECSRHT_TEST_PG is unset, so the pure-logic half — batch
duplicate detection, token comparison, argument guards, and the agreement
between schema.sql and migrations/0001_initial.sql — still runs with no
database available.