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/adr/README.md

@@ -17,3 +17,4 @@ This directory contains Architecture Decision Records (ADRs) for josh-sync. Each
 | [009](009-tree-comparison-guard.md) | Tree comparison as sync skip guard | Accepted |
 | [010](010-onboard-checkpoint-resume.md) | Onboard workflow with checkpoint/resume | Accepted |
 | [011](011-linearize-fallback-reverse.md) | Linearize fallback for reverse sync | Accepted |
+| [012](012-explicit-monorepo-url.md) | Explicit `monorepo_url` (drop first-target-host inference) | Accepted |