docs: adapt SDD process + agent specs for alkgit

- sdd_process.md: alkcall -> alkgit
- coordinator: spawn prompt targets alkgit, constraints point at AGENTS.md
- implementation-specialist: conventions rewritten for the git-server
  workspace (replaces alkcall-specific vendored-types/BAST/abort items)
- code-reviewer: thiserror (no anyhow), sha256/acme feature flags
- architect/architecture-reviewer: examples de-alknetted

Verified: none needed (markdown only)

.opencode/agents/architecture-reviewer.md

@@ -158,8 +158,9 @@ blocks downstream. Check:
 
 - Is the impacts field present? Absence is a warning — without it, the
   deferral's urgency is invisible and triage is guesswork.
-- Is it specific? "Blocks the first hub deployment because the hub
-  dials workers" is useful. "Blocks the hub crate" is boilerplate.
+- Is it specific? "Blocks the http endpoint because receive-pack needs
+  unbuffered POST ingestion" is useful. "Blocks the http crate" is
+  boilerplate.
 - Does it match the priority? A `deferred(unclear)` with high priority
   but a vague impacts field ("blocks future features") is a mismatch —
   if it's high priority, it blocks something specific; say what.