Release 2.0.0 — explicit monorepo_url, drop first-target-host inference

Breaking change. The monorepo's gitea host was previously inferred from the
first target's subrepo_url, silently coupling the two and breaking on any
multi-host setup (monorepo on host A, target on host B). v2.0.0 replaces the
inference with a required josh.monorepo_url field that mirrors subrepo_url —
same parsing, same SSH/HTTPS auth options, same shape.

- schema_version: 2 is now required; v1 configs are rejected with a clear migration error
- josh.monorepo_url is required; josh.monorepo_auth is optional (default https)
- mono_auth_url() in lib/auth.sh mirrors subrepo_auth_url()
- MONOREPO_API derives from monorepo_url's host instead of .[0].gitea_host;
  env-var override preserved as escape hatch
- initial_import() and force-push call mono_auth_url() instead of building URLs inline
- ADR-012 documents the rationale; CHANGELOG includes migration snippet

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/guide.md

@@ -78,9 +78,13 @@ git ls-remote https://josh.example.com/org/monorepo.git HEAD
 Create `.josh-sync.yml` at the monorepo root. Each target maps a monorepo subfolder to an external subrepo:
 
 ```yaml
+schema_version: 2
+
 josh:
   proxy_url: "https://josh.example.com"     # josh-proxy URL (no trailing slash)
-  monorepo_path: "org/monorepo"              # repo path as josh sees it
+  monorepo_path: "org/monorepo"              # repo path as josh-proxy sees it
+  monorepo_url: "git@gitea.example.com:org/monorepo.git"  # monorepo git clone URL
+  monorepo_auth: "ssh"                       # optional, default "https"
 
 targets:
   - name: "billing"                          # unique identifier