docs: make the user guide self-contained Starlight source

[davideimola]

docs/guide/ becomes content source for a Starlight site living in a separate repository, rather than documentation meant to be read on GitHub. Where the two readers conflict, Starlight wins — that trade-off, and the reasoning behind it, is recorded in ADR 0022.

What changed

• 18 relative links removed — 15 into docs/adr/, one to CONTEXT.md, one to a design doc, one to the Dockerfile. The rationale a user actually needs is absorbed into prose in their own language; the rest is cut. Nothing in the guide now points at an ADR, a design document or a source path.
• Frontmatter on every page (title, description, sidebar.order), body <h1>s dropped since Starlight renders the heading from title. Ordering is all-or-nothing in an autogenerated sidebar — a page without order sinks to Number.MAX_VALUE — so all eight declare one.
• README.md → index.md, the section landing page.
• 18 internal cross-references rewritten as absolute /guide/… paths, because Astro never rewrites relative .md links.
• Blockquote callouts → typed asides: caution for security and spend, tip for recommendations, note for version and migration notes.
• Domain vocabulary glossed inline (Person, SOUL, MEMORY) now that CONTEXT.md is unreachable from here, and Argus's Service principal disambiguated from the Kubernetes Service throughout the hosting guide.
• Dropped a cp pkg/skill/builtin/... command that only worked for somebody who had cloned the repo.

AGENTS.md states the rule so it survives; the CONTEXT.md Service entry gains the Kubernetes disambiguation.

Requirements this places on the site repository

All three are load-bearing and none can be satisfied from this repo:

1. Mount the folder at src/content/docs/guide/. Starlight's docsLoader() hard-codes its base to <srcDir>/content/docs and does not expose it, so the folder must physically appear there — submodule or CI sync. Mounted elsewhere, every internal link breaks silently.
2. Relabel the sidebar groups in astro.config.mjs. An autogenerated group takes the directory name verbatim, so channels/ and deployment/ render lowercase. No _meta.json, no frontmatter override, no per-directory config can fix that from the content side.
3. Serve from the domain root. The internal links are root-absolute and hand-written; Astro does not inject its base into those, so a site deployed under a sub-path breaks all of them.

The Starlight conventions relied on here were verified against Starlight 0.41.5 / Astro 7.1.6, with sources pinned to commit SHAs, in docs/research/starlight-content-conventions.md.

Known costs, accepted

• docs/guide/ no longer reads well on GitHub: internal links point at site slugs, :::caution shows as literal text, and the folder has no landing page because GitHub does not render index.md.
• Some rationale now exists twice — once in an ADR, once as guide prose — and the two can drift. Nothing enforces the rule but the AGENTS.md section; a CI check would be a grep and was deliberately left out to keep this change scoped to documentation.

Follow-ups, not addressed here

• No documented way to fork a built-in skill. Removing the cp leaves a gap: argus skill ls shows only name and description, and the body is not exportable from the CLI. Closing it needs a command (argus skill export), not a doc change.
• Two claims in skills.md contradict CONTEXT.md. The page states that skills are an analyst+ capability viewers cannot enumerate, and that RBAC is enforced at the tool layer. The glossary says list_skills/read_skill sit in every Session's registry whatever the Role, and calls Tool-layer RBAC a no-op today. Left untouched: correcting them means publicly documenting an unenforced control, which is a communication decision rather than a typo.