feat: build system hardening — collision detection, validation gates, init path enforcement

5-phase hardening to prevent silent file-layer collisions (the D-Bus
regression class):

Phase 1: lint-config-paths.sh + make lint-config in depends.mk
Phase 2: CollisionTracker in installer (content-hash comparison)
Phase 3: installs manifests in recipe.toml + validate-file-ownership.sh
Phase 4: validate-init-services.sh + make validate in disk.mk
Phase 5: documentation (AGENTS.md, BUILD-SYSTEM-HARDENING-PLAN.md)

Both redbear-mini and redbear-full build and validate clean.
66 declared install paths in base, zero conflicts.

config/redbear-netctl.toml

@@ -90,7 +90,7 @@ IP=bounded
 """
 
 [[files]]
-path = "/usr/lib/init.d/12_netctl.service"
+path = "/etc/init.d/12_netctl.service"
 data = """
 [unit]
 description = "Network profile application"