refactor: one derivation of a proposal's branch name, in core

db.ProposalBranch and gitx.ProposalBranch each spelled out "proposals/" +
id with different signatures and different error behaviour: db formatted
anything including 0, gitx refused a non-positive id. Two derivations of
one name is a proposal whose row and whose ref can disagree, which is
painful to trace and cheap to prevent.

core now owns ProposalPrefix and ProposalBranch(int64) (string, error).
gitx.ProposalPrefix and db.BranchPrefix are that constant, gitx wraps
core's error in ErrBadRev so its callers keep their failure class, and
db.ProposalBranch delegates — inheriting core's refusal of a non-positive
id, which is why its signature grew an error.

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.

feat: foundation — go.mod with every dependency, and the core/ domain

Phase 1 foundation commit. Two things, so that later parallel waves write
disjoint directories and never touch go.mod:

  - go.mod / go.sum carrying every external dependency the whole module will
    need (go-git, bleve, goldmark, chi, lib/pq, yaml.v3, the MCP SDK, brant,
    auxilia, testify, and the sr-ht-core fork). Populated by building a
    throwaway blank-import file, which is then deleted; `go mod tidy` runs
    once, at the very end of the build-out.
  - core/, the pure domain: owner/space names, safe relative paths, the
    globally-unique document ID grammar, frontmatter parsing and schema
    validation, `.spec.yml` policy with auto_merge glob matching, and the
    proposal state machine. Standard library plus yaml.v3, nothing else.

Two design invariants are enforced here rather than documented and hoped for:
"approved" is not a status (it is a property of the branch a document is
reachable from), and the proposal machine has exactly open/merged/rejected.
A per-space `.spec.yml` cannot reintroduce either.

Note on the sr-ht-core pin: the design calls for a `replace` onto
git.srht.bigb.es/~bigbes/core-go at c2c2f38, but that commit's go.mod still
declares `module git.sr.ht/~sircmpwn/core-go`, so Go rejects the replacement.
Both siblings pin the later dd418a20 under the canonical path with no
replace; this does the same.