Add versioning contract: semver policy, breaking change sections, schema_version guard

- CHANGELOG: add ### Breaking Changes section to all versions (None for 1.1–1.3)
- README: add Versioning section with semver table and floating tag explanation
- schema/config-schema.json: add schema_version field (const: 1, optional)
- lib/config.sh: fail fast if schema_version is present and unsupported

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## 1.3.0
 
+### Breaking Changes
+
+_None._
+
 ### Features
 
 - **Linearize fallback for reverse sync**: When josh-proxy rejects a push due to unmappable merge commits, reverse sync automatically falls back to linearizing the history — cherry-picking regular commits individually and squashing only the problematic merge commits via `cherry-pick -m 1`. Preserves individual commit granularity while handling complex merge topologies that josh cannot map. PR body notes when linearization was used.
@@ -19,6 +23,10 @@
 
 ## 1.2.0
 
+### Breaking Changes
+
+_None._
+
 ### Features
 
 - **File exclusion**: `exclude` config field removes files/directories from the subrepo at the josh-proxy transport layer. Patterns are embedded inline in the josh-proxy URL using `:exclude[::pattern,...]` syntax — no extra files to generate or commit.
@@ -38,6 +46,10 @@
 
 ## 1.1.0
 
+### Breaking Changes
+
+_None._
+
 ### Features
 
 - **`onboard` command**: Interactive, resumable workflow for importing existing subrepos into the monorepo. Walks through: prerequisites check, import (creates PRs), wait for merge, reset (pushes josh-filtered history). Checkpoint/resume at every step.