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.

AGENTS.md

@@ -144,6 +144,51 @@ make all
 - **Syscall ABI**: Unstable intentionally. Stability via `libredox` and `relibc`
 - **Drivers**: ALL userspace daemons via scheme system. No kernel-space drivers (except serio)
 
+## INSTALLER FILE LAYERING
+
+The installer creates filesystem images in four layers. Understanding this ordering is critical
+to avoid silent file overwrites.
+
+### Layer Ordering During `install_dir()`
+
+```
+Layer 1: Config pre-install [[files]]    (postinstall = false)
+Layer 2: Package staging                  (install_packages())
+Layer 3: Config post-install [[files]]    (postinstall = true)
+Layer 4: User/group creation              (passwd, shadow, group)
+```
+
+### Collision Implications
+
+- **Layer 2 overwrites Layer 1** silently (same path → last writer wins). This is the bug class
+  that caused the D-Bus regression: config overrides at `/usr/lib/init.d/` were overwritten by
+  the `base` package staging the same paths.
+- **Layer 3 overwrites Layer 2** (intentional — postinstall overrides).
+- For init services, config overrides **MUST** use `/etc/init.d/` so they survive Layer 2.
+
+### Init Service File Ownership
+
+- **Packages own `/usr/lib/init.d/`** — default service files installed by recipe staging
+- **Config overrides own `/etc/init.d/`** — override files created by `[[files]]` entries
+- The init system's `config_for_dirs()` gives `/etc/init.d/` priority via BTreeMap dedup
+- **Config `[[files]]` entries MUST NOT use `/usr/lib/init.d/` paths for init services**
+- Run `make lint-config` to detect violations
+
+### Collision Detection
+
+The installer now includes a `CollisionTracker` (in `collision.rs`) that detects when package
+staging overwrites config pre-install files. Init service collisions always error. Other
+collisions warn by default, error in strict mode (`REDBEAR_STRICT_COLLISION=1`).
+
+### Validation Gates
+
+After building an image, run `make validate` to verify:
+- Init service path violations (via `lint-config`)
+- Override effectiveness and scheme binary existence (via `validate-init-services.sh`)
+- File ownership conflicts (via `validate-file-ownership.sh`)
+
+See `local/docs/BUILD-SYSTEM-HARDENING-PLAN.md` for the full plan.
+
 ## ANTI-PATTERNS (THIS PROJECT)
 
 - **DO NOT** suppress errors with `as any` / `@ts-ignore` — use proper `Result` handling