feat: manifest driven template engine

[f0oster]

Replace the hardcoded parameter, prompt, and validation logic with a manifest-driven system.

New-AnvilModule gains a -Template parameter for scaffolding from custom templates. Users can point it at any directory containing a valid template.psd1 and the engine will prompt for parameters, validate input, apply conditions, and produce the output. The built-in nodule template is now an instance of the same system.

The scaffold template configuration has been (mostly) decoupled from Anvil's source code as a result. The template directory contains template.psd1 which declares the template parameters, conditions, sections, and layers.

Template engine changes:

• Invoke-TemplateEngine supports IncludeWhen/ExcludeWhen file conditions and conditional content sections via <%#section%> markers
• Module template configuration moved to template.psd1 manifest
• Token replacement renamed from ModuleName to Name throughout templates
• File/license/docs exclusion handled by manifest conditions instead of post-processing deletes
• CI provider layering driven by manifest Layers declaration
• Docs task conditionally included via template sections
• Get-AnvilTemplate reads manifest metadata and discovers layers from manifest declarations rather than scanning for a CI directory
• .gitattributes added to template for consistent line endings
• Git init no longer stages and commits automatically
• Git init and Docs are now enabled by default.

New private functions:

• Test-ManifestCondition, Test-FileCondition (condition evaluation)
• Resolve-TemplateSections (conditional content blocks)
• Format-TokenValue, Resolve-AutoToken (token formatting pipeline)
• Invoke-ManifestPrompt, Assert-ManifestConfiguration (manifest-driven prompt and validation, replacing Invoke-InteractivePrompt and Assert-ValidConfiguration)
• Read-TemplateManifest, Assert-TemplateManifest (manifest loading and schema validation)
• Convert-PromptResult, Resolve-DefaultFrom, Read-PromptUri (helpers)

New public functions:

• Invoke-AnvilBuild (wrapper around InvokeBuild, resolves project root)

Removed:

• Invoke-InteractivePrompt, Assert-ValidConfiguration, Copy-CITemplates
• getting-started/ directory no longer ships with scaffolded projects

Documentation:

• Consolidated docs and readme to flow better
• Rewrote scaffolded project README
• Added template-authoring.md guide
• Updated all user-facing references to use Invoke-AnvilBuild
• Added CompatiblePSEditions to module manifest template