Remove astro-icon integration

[davidsneighbour]

Follow-up from Dependabot alert 424 / the extract-zip fix.

The site currently renders Bootstrap, Lucide, Simple Icons, and Font Awesome brand icons through astro-icon. The security fix removed the vulnerable extract-zip path by overriding @iconify/tools, but we should remove the astro-icon integration entirely instead of keeping that compatibility layer long-term.

Target state:

• Remove astro-icon from astro.config.ts and package metadata.
• Remove @iconify/tools and all @iconify-json/* packages.
• Stop copying Bootstrap Icons into src/icons/; remove the generated Bootstrap icon workflow if it is no longer needed.
• Use only Lucide and Simple Icons going forward.
• Replace existing Font Awesome brand icons with Simple Icons equivalents.
• Migrate Bootstrap-style bare icon names to Lucide equivalents where possible.
• Keep a small explicit icon registry only where dynamic icon names from JSON/content still require indirection.
• Update the icon documentation and agent instructions so new work uses Lucide for UI icons and Simple Icons for brands.

References:

• Lucide Astro guide: https://lucide.dev/guide/astro/getting-started
• Simple Icons Astro: https://github.com/dzeiocom/simple-icons-astro

Known affected areas:

• astro.config.ts
• src/components/shared/links/IconLink.astro
• src/components/shared/elements/Button.astro
• components importing astro-icon/components
• src/content/social.json
• tag frontmatter using simple-icons:*
• src/utils/icon-names.ts
• src/scripts/maintenance/create-icon-types.ts
• src/packages/site/astro.jsonc
• src/packages/site/icons.jsonc
• documentation/theme/icons.md
• .agents/instructions/src-components-astro.instructions.md

Acceptance criteria:

• rg "astro-icon|@iconify|fa7-brands|bootstrap-icons" shows no runtime dependency on the old icon pipeline, except for historical documentation if deliberately retained.
• npm ls extract-zip remains empty.
• npm test passes.
• npm run check passes.