docs: document ripgrep and ImageMagick as external tools; add both to Doctor

[elhoim]

GETTING-STARTED.md is the page that lists the external tools doctrine uses, and it opens with a promise: "None are required — every one degrades honestly when absent." Two tools that shipped code shells out to were missing from it, and from Doctor's registry.

ripgrep

ContextSearch/Tools/ContextSearch.ts spawns rg directly, as do the work sweep and UpdateModels.ts's drift scan. LifeOS indexes through the filesystem instead of a vector store, so this is load-bearing rather than a style preference — the architecture doc says as much ("fast search such as with Ripgrep can give us everything people usually want from a RAG system").

Worth separating: the built-in Grep tool is independently ripgrep-backed and unaffected. This is only about the tools that spawn rg themselves.

ImageMagick

Interceptor's blank-frame guard and Art's composition steps both spawn magick.

Capture.sh already handles absence well — it returns the screenshot but prints BLANK-FRAME GUARD SKIPPED (imagemagick-absent) — do not treat it as pixel-verified without looking, with an install hint. That is exactly the honest degradation the page promises. The gap was that nothing told you beforehand, so the first signal was a warning mid-verification.

Changes

• Both added to Doctor's capability registry, so they surface as ✅/❌ with a fix command like every other tool, and can be declined.
• Both documented in GETTING-STARTED.md in the same shape as the existing entries, with the ImageMagick note recording what actually happens when it is absent.

Verified

Doctor.ts run with both binaries present → both report ✅ live. The same which() logic run against a PATH with rg and magick stripped → both report broken and surface their fix command. Doctor's existing capabilities are unchanged in both runs.