docs: normalize per-image README headings and shared sections

[gatezh]

What

Normalizes the per-image READMEs in place (option a from #132). Headings are now plain text, and the shared sections have one name and one order. The rule is recorded in .claude/CLAUDE.md.

Why

The READMEs had drifted apart: four used plain headings and two used emoji, and the same concept appeared under different names (Multiplatform Support/Multi-platform Support, Version Information/Build Args, Quick Start/Usage). Each README stays self-contained so it still reads correctly on its own, e.g. on the ghcr package page.

Changes

• Canonical order: Features → Multi-platform Support → Image Tags → Usage → image-specific sections → Build Args → Building the Image → Resources → License → Contributing. Each name is the one most READMEs already used.
• hugo-bun, hugo-bun-node, root README.md: removed emoji from headings
• bun, hugo-bun, hugo-bun-node: renamed Version Information → Build Args (the section lists ARG defaults, and Build Args is claude-code's name for it), reordered, and added a two-line Image Tags section derived from each build workflow
• claude-bun: renamed Quick Start → Usage, moved Image Tags up, and added Multi-platform Support (its workflow builds amd64 and arm64)
• ralphex-fe: renamed Building Locally → Building the Image and added Multi-platform Support
• .claude/CLAUDE.md: one Code Style bullet stating the rule

Notes

• Per-image tag schemes are unchanged. They differ on purpose (see the Per-image READMEs have drifted in heading style and duplicated boilerplate #132 table).
• To avoid conflicts with open PRs, a few spots don't fully follow the rule yet:
  • claude-code/README.md is untouched (feat(claude-code): bake happy CLI into default and sandbox images #131, feat(claude-code): upgrading-dependencies skill; sync skill 1.2.0 #136 and feat(images): provide MDN MCP server via managed settings #153 edit it). It still has Quick Start.
  • ralphex-fe is not reordered, because feat(images): provide MDN MCP server via managed settings #153 edits lines around ## Usage.
• No heading anchors were referenced by in-repo links or the wiki, so no links needed fixing.
• Content accuracy was out of scope. These stale facts turned up and are left for a follow-up:
  • hugo-bun lists Hugo 0.152.2 / Bun 1.3.2; the Dockerfile has 0.155.1 / 1.3.8.
  • hugo-bun-node lists Node 24.13.0; the Dockerfile has 24.21.0.
  • ralphex-fe's Tools table and tag example are several versions behind.
  • The bun and hugo-bun* examples use ghcr.io/<USERNAME>/… rather than gatezh.
  • .github/copilot-instructions.md is a broken symlink to a nonexistent AGENTS.md.

Fixes #132