docs: rewrite documentation for density, prep v1.2.0

[corvid-agent]

Summary

• README rewritten — 780 deletions, 342 insertions. Every line carries new information, no filler or redundancy.
• Docs site streamlined — pages complement the README instead of duplicating it.
• CHANGELOG backfilled — added missing v1.1.1 and v1.1.2 entries.
• Version bumped to 1.2.0 in Cargo.toml, ready for release.

What changed

File	Change
README.md	Cut filler, merged redundant sections, tightened every table and description
CHANGELOG.md	Added 1.1.1 (marketplace fixes), 1.1.2 (merge conflict + CI fixes), 1.2.0 (this PR)
Cargo.toml	version = "1.2.0"
docs/index.md	Focused landing page, links to detail pages instead of duplicating them
docs/spec-format.md	Added "What SpecSync checks" column to required sections table
docs/cli.md	Removed duplicate examples, tighter command descriptions
docs/configuration.md	Consolidated per-option sections into single table
docs/github-action.md	Removed redundant prose around code blocks
docs/ai-agents.md	Cut fluff, updated minimal spec example date
docs/architecture.md	Compressed design principles into direct statements

Test plan

• Verify docs site renders correctly at corvidlabs.github.io/spec-sync
• cargo publish --dry-run succeeds with new version
• Tag v1.2.0 after merge

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