feat(core): add six GEO surface checks and tighten meta/image parsing

[antlio]

Summary

• Adds six new GEO audit checks: https-enabled, meta-description, open-graph-tags, external-citations, substantial-text-content, image-alt-text
• Tightens shared HTML parsing: extractMetaTags keys are lowercased so capitalized attribute names match; extractImages reads attributes with matching quote types so apostrophes inside double-quoted alt values survive
• CLI derives networkSkipped from ALL_CHECKS.length instead of a hard-coded constant
• image-alt-text excludes decorative alt="" from the denominator and compares the raw ratio (not the rounded percent) against thresholds
• meta-description no longer falls back to og:description; whitespace-only content is treated as missing; the warn message surfaces missing-tag counts and escalates to fail at ≥50% missing
• open-graph-tags message surfaces partial and none-covered counts (previously metadata-only)
• https-enabled declared requiresNetwork: true and weight reduced from 1.0 to 0.7 to stop dominating url-stability scoring; restores pre-diff local-mode category shape

Test plan

• bun run typecheck — green
• bun run --filter @agentimization/core test — 38/38 pass
• bun run build — green
• Run npx agentimization https://yoursite.com against a known-good site and confirm the new checks fire with sane scores
• Run npx agentimization ./ against a local repo and confirm url-stability no longer appears in result.categories (network-only check filtered out)