Add Portable AI install and smoke guide on current main

Author: mdheller

docs/install.md

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+# SourceOS Devtools install and smoke guide
+
+This guide covers the current Portable AI Kit scaffold in `sourceos-devtools`.
+
+## Local source checkout
+
+Run from the repository root:
+
+```bash
+python3 bin/sourceosctl portable-ai profiles
+python3 bin/sourceosctl portable-ai preflight /tmp/SOURCEOS_AI --profile tiny-router
+python3 bin/sourceosctl portable-ai prepare /tmp/SOURCEOS_AI --profile tiny-router --dry-run
+python3 bin/sourceosctl portable-ai start-plan /tmp/SOURCEOS_AI --provider ollama-compatible --surface turtleterm
+python3 bin/sourceosctl portable-ai stop-plan /tmp/SOURCEOS_AI --provider ollama-compatible
+python3 bin/sourceosctl portable-ai byom verify /tmp/SOURCEOS_AI ./models/example.gguf --name example
+```
+
+The default posture is evidence-first and non-mutating. Prompt egress is denied by default. Tool use is denied by default. Runtime start and stop commands emit plans; they do not launch or stop a provider.
+
+## Homebrew scaffold
+
+Before tap promotion, use the repository-local formula for syntax and smoke validation only:
+
+```bash
+brew install --HEAD https://raw.githubusercontent.com/SourceOS-Linux/sourceos-devtools/main/packaging/homebrew/Formula/sourceos-devtools.rb
+```
+
+After tap promotion:
+
+```bash
+brew install SourceOS-Linux/tap/sourceos-devtools
+```
+
+## Validation
+
+```bash
+python3 -m unittest discover -s tests -v
+make validate
+python3 scripts/validate_packaging.py
+```
+
+The packaging scaffold must not download model weights, call external providers, start local runtimes, store prompt bodies, or bypass Agent Machine / policy-gated runtime activation.