docs: reorganize into DOC/ and simplify README

- Move all docs (DEV, AGENT_Context, algorithm) into DOC/ via git mv
- Rewrite root README as short intro + quick start + links into DOC/
- Add per-area reference docs: SCRIPTS (core scripts + libs + features),
  POS (pos CLI + compose config), APPS (picker + catalog), SYSTEMD (units)
- Add DOC/README.md index; fix all cross-references

DOC/README.md

@@ -0,0 +1,20 @@
+# Documentation
+
+Everything in this folder is reference material for the `Linux_post_install` project. The root [README](../README.md) is the short intro + quick start; this folder holds the detail.
+
+| Document | What it covers |
+|----------|----------------|
+| [SCRIPTS.md](SCRIPTS.md) | Core installer scripts: `install.sh`, `preinstall.sh`, `postinstall.sh`, `lib/common.sh`, `lib/flags.sh`, `features/autostart.sh` — purpose, how each works, configuration |
+| [POS.md](POS.md) | The `pos` CLI: dispatcher, every `pos-*` command, Docker Compose / ScaleTail config, legacy wrappers, flag CLIs |
+| [APPS.md](APPS.md) | Optional apps: `apps/install.sh` picker, installer conventions, full app catalog |
+| [SYSTEMD.md](SYSTEMD.md) | Systemd units (`autostart.service`, `ssh-agent.service`), feature-flag gating, bash completion |
+| [DEV.md](DEV.md) | Developer guide: architecture, conventions, how to add tools/apps/features, commit guidelines |
+| [AGENT_Context_Project.md](AGENT_Context_Project.md) | Single-source context doc for AI agents working on the repo |
+| [algorithm.md](algorithm.md) | ASCII diagrams: install flow, `pos` dispatch, compose `up`, config cascade, logging, vbox lifecycle |
+
+## Quick navigation
+
+- Just installed and want to use it? → [POS.md](POS.md)
+- Adding a package? → [SCRIPTS.md → preinstall.sh](SCRIPTS.md#preinstallsh--system-packages)
+- Adding a CLI tool or app? → [DEV.md](DEV.md)
+- First deploy of a self-hosted service? → [POS.md → Docker Compose](POS.md#docker-compose--scaletail)