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  (http router factory); both marked Superseded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).

.opencode/agents/architect.md

@@ -329,7 +329,7 @@ A decision should be `deferred(scope)` when:
 - The use case isn't concrete (e.g., "we don't know what the admin API
   will need from the call protocol")
 - The options depend on something that doesn't exist yet (e.g.,
-  "depends on the alkgit-http streaming adapter shape")
+  "depends on the alkhttp git-feature shape")
 - The trade-off requires data that can only come from implementation
   (e.g., "need performance benchmarks to choose between X and Y")
 - The decision is genuinely not needed for the current scope (e.g., "the

.opencode/agents/coordinator.md

@@ -206,11 +206,12 @@ Your task: {{task}}
 8. Notify: worktree({action: "notify", args: {message: "Task completed: {{task}}. <brief summary>", level: "info"}})
 
 Key project constraints (@alkdev/alkgit):
-- Rust workspace: crates/alkgit-core, -transport, -http, -ssh, alkgitd
+- Pure protocol crate: single `alkgit` at repo root (ADR-010); no
+  workspace, no binary, no front-door crates
 - Rust: use cargo build, cargo clippy, cargo fmt, cargo test
 - No comments in code
 - thiserror for library error types; no panics in library code
-- Feature flags for optional surface (sha256, acme)
+- Feature flags for optional surface (gix default-on, sha256)
 - Async via tokio runtime
 - gitoxide (gix) for git storage/wire primitives; never shell out to git
 ```