refactor(architecture): ADR-010 — pure protocol crate (alktty template)

Structural decision (OQ-09 resolved): alkgit follows the alktty/
alktunnels template — a single published protocol crate on alkcall
channels, no binary, no front doors.

- ADR-010 supersedes ADR-001 (crate decomposition) and ADR-006
  (http router factory); both marked Superseded
- Single crate at repo root: Cargo.toml with gix feature (default-on
  backend implementations; wire layer compiles without it —
  gix-hash always-on with sha1 per the compile-time-rejected
  invariant), crates/ workspace deleted, src/lib.rs stub in place
- doors.md replaces http.md/ssh.md/alkgitd.md: alkhttp git-feature
  sequencing (after first publish), alkssh requirement (fixed-grammar
  exec dispatch), native alk/git path, downstream assembly
- backend.md replaces storage.md: GitRegistry/GitRefs/GitPackGen/
  GitPackIngest traits (ingest validates, refs commits — single CAS
  home), gix feature encodes POC-2 prerequisites
- transport.md reframed for the single crate; backend traits replace
  hook traits in the public API
- OQ-09 resolved (all five sub-decisions in ADR-010), OQ-01 resolved
  (subsumed), OQ-03 narrowed to publish-freeze, OQ-08 narrowed to
  registry identity + vault placement, OQ-07 rescoped to the gix
  feature's registry impl
- vision.md v2: single-binary/monorepo framing corrected as
  init-agent artifact; POC checklist marked complete
- AGENTS.md + .opencode agent specs updated to the new shape

Verification: cargo build (default + no-default-features), cargo test
--all-features, clippy --all-features -D warnings, fmt --check all
pass. Third review round: zero critical, all warnings/suggestions
addressed (GitPackGen signature amended in ADR-004, stale anchors
fixed, ADR-006 body tense normalized, CAS split stated, vision
residuals cleaned).

docs/architecture/decisions/008-registry-resolved-repo-identity.md

@@ -19,7 +19,7 @@ Wire-supplied repo names are **IDs**: opaque registry keys resolved
 server-side to configured storage roots.
 
 - The registry is alkgit's authoritative (repo id → storage root +
-  visibility + ACL scope) mapping, owned by `alkgit-core`.
+  visibility + ACL scope) mapping, owned by the `GitRegistry` backend trait.
 - Resolution failure and authorization failure are indistinguishable to
   the caller (ADR-007 step 2's no-existence-oracle rule).
 - Wire names are never joined, normalized, or canonicalized into paths.
@@ -43,4 +43,4 @@ server-side to configured storage roots.
 - `docs/research/git-protocol.md` §"Security-relevant protocol notes"
 - `docs/research/vision.md` §"Immediate threat-model notes"
 - ADR-007 (the resolve-then-authorize order)
-- storage.md §registry, OQ-06 (registry backing)
+- backend.md §"The trait family" (GitRegistry), OQ-06 (registry backing)