docs: OSS-style README, brand banner, and user guide under docs/guide

[davideimola]

What

Reworks the repo's front door in the style of well-known OSS dev tools (fastify, gin, cobra, charm, kubernetes) and stands up a task-oriented user guide.

README

• Centered brand banner (<picture>, light/dark variants) replaces the H1; tagline + badge row (CI, release, Go Reference, Go Report Card, license — the Go/pkg.go.dev badges light up once the repo is public).
• Flow: what Argus is → why it exists → compact quickstart → features → channel table → built-in skills → docs links → pre-1.0 status.
• Reference material (full config, command table, upgrade notes, project layout) moved out to the guide; no internal companion docs cited.
• Fixes a factual error: Slack was presented as an existing channel — only github/mcp/tui exist today, so it is now marked planned.

Brand

• brand/argus-banner-{light,dark}.svg: feather + wordmark + "application security agent" lockup, system fonts only (no external assets).

Docs restructure (by audience)

• docs/guide/ (new, user-facing): getting started, argus.yaml reference, GitHub channel (auto reviews, bare-name vocative, security posture), MCP channel, skills guide, and deployment/kubernetes.md moved in with links fixed.
• docs/design/: authz-skill-design*.md moved out of the docs root.
• docs/skills/ removed: the authz-audit companion README duplicated the design doc and the new skills guide — and it can't live next to the skill in pkg/skill/builtin/ because //go:embed all:builtin would bundle it into the binary as a skill file. User-facing content now lives in docs/guide/skills.md.
• CONTEXT.md, docs/adr/, docs/agents/ untouched (paths fixed by the grilling conventions).

Verification

• Script-checked that every relative markdown link in the repo resolves.
• Banner SVGs rendered and visually verified in both light and dark variants.

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