feat: meta.json hidden field for URL-routable-but-sidebar-excluded pages (0.1.2-alpha)

[Shewart]

Summary

Adds "hidden" support to meta.json and the underlying NavigationGraph. Enables a docs site to have pages that route (URLs resolve, direct links work, package-selector navigation works) but never appear in the sidebar tree.

Dogfood-driven: surfaced building shelldocs.dev where package landing pages need to be reachable via the sidebar package selector without also cluttering the lower sidebar with a redundant "Packages" section.

The API

{
  "title": "Documentation",
  "pages": ["introduction", "getting-started"],
  "hidden": ["components", "cli", "markdown", "core", "tokens"]
}

Slugs (or folder names) in hidden are excluded from:

• Explicit pages rendering (if double-listed, hidden wins)
• Auto-append tail (0.1.1's default behavior for unreferenced files)
• Prev/Next walking (GetPrevNext skips them)
• Flatten() iteration

But remain URL-routable: NavigationGraph.ResolveByUrl("/hidden-page") returns the node, direct links + package-selector navigation work.

How it works under the hood

NavigationGraphBuilder.BuildFolder now threads a hidden collection through recursion. When meta.Hidden seeds a slug, the corresponding page (or folder-subtree, wrapped in a synthetic section node so Index() can recurse it) is pushed into that collection. NavigationGraph constructor accepts an optional hiddenPages param and indexes them into _byUrl only — never into _flatPages, never into Root.Children.

Release chore bundled

Rather than splitting into two branches, folded the release chore in:

• Directory.Build.props: <Version>0.1.2-alpha</Version>
• src/ShellDocs.CLI/Commands/InitCommand.cs: ShellDocsVersion const bumped alongside (per the dual-bump note in docs/RELEASING.md)
• CHANGELOG.md: [0.1.2-alpha] entry + compare-link footer

Post-merge, git tag v0.1.2-alpha && git push origin v0.1.2-alpha publishes cleanly through the existing Trusted Publishing workflow.

Test plan

• dotnet build shelldocs.slnx — clean, 0 warnings, 0 errors
• dotnet test shelldocs.slnx — 147 / 147 passing (+4 new in NavigationGraphBuilderTests)
• bash scripts/pack-dry-run.sh — all 6 packages pack cleanly at 0.1.2-alpha

New tests (NavigationGraphBuilderTests)

• Hidden slug excluded from sidebar but URL resolves via ResolveByUrl
• Hidden folder excluded from sidebar but each child URL still resolves
• hidden takes precedence over pages (slug in both stays hidden)
• Hidden slug excluded from the auto-append tail (was the corner case that surfaced the whole need)

Files touched

• src/ShellDocs.Core/MetaJson.cs — new Hidden field
• src/ShellDocs.Core/NavigationGraph.cs — optional hiddenPages constructor param + IndexHidden helper
• src/ShellDocs.Core/NavigationGraphBuilder.cs — BuildFolder threads a hidden list, seeds from meta.Hidden, wraps hidden folders in synthetic sections for URL indexing
• tests/ShellDocs.Tests/NavigationGraphBuilderTests.cs — 4 new tests
• Directory.Build.props, src/ShellDocs.CLI/Commands/InitCommand.cs, CHANGELOG.md — release chore