feat: Initialize-GitDefaults.ps1 -- compose .gitattributes/.gitignore from upstream templates

[MarkMichaelis]

Goal

Ship Initialize-GitDefaults.ps1 (repo root) that interactively (or via -Language) composes .gitattributes and .gitignore for a consumer project by combining per-language templates from upstream community repos plus a curated PowerShell block.

Replaces shipping a static .gitattributes.template from upstream IntelliSDLC.ai. Language stacks vary per consumer; the per-language templates already exist upstream -- we just need to compose them.

Source Repos (Pinned)

• .gitignore: github/gitignore @ dcc0fc7bc2b5ba480cf117ad1be31bafceeaff46 (GitHub-org authoritative; powers GitHub's UI picker).
• .gitattributes: alexkaratarakis/gitattributes @ fddc586cf0f10ec4485028d0d2dd6f73197a4258 (community de facto; github/gitattributes does not exist).

Ecosystem Background

Tool	.gitignore	.gitattributes
gibo (simonwhitaker/gibo)	yes -- fetches from github/gitignore	no
gh repo create --gitignore X	yes -- creation-time only	no
Any other CLI	no	no

Initialize-GitDefaults.ps1 integrates with gibo when present (for .gitignore) and fills the ecosystem gap for .gitattributes ourselves.

Required Language Coverage

Language	gitattributes	gitignore
CSharp	CSharp.gitattributes	VisualStudio.gitignore
PowerShell	curated in-script block	curated in-script block
TypeScript	Web.gitattributes	Node.gitignore
ASP.NET	+VisualStudio.gitattributes (layered on CSharp)	VisualStudio.gitignore (shared)

Dependency graph: ASP.NET -> CSharp. TypeScript uses Web + Node directly.

Bundled Offline Snapshots

Under .github/templates/git-defaults/ at pinned SHAs:

• From alexkaratarakis/gitattributes: Common.gitattributes, CSharp.gitattributes, VisualStudio.gitattributes, Web.gitattributes
• From github/gitignore: VisualStudio.gitignore, Node.gitignore, Global/Backup.gitignore
• SOURCES.md documenting pinned SHAs + authority distinction + refresh procedure.
• PowerShell blocks NOT bundled -- embedded in script.

Script Spec -- Initialize-GitDefaults.ps1

[CmdletBinding(SupportsShouldProcess)]
param(
    [string[]] $Language,
    [switch]   $IncludeGitignore     = $true,
    [switch]   $IncludeGitattributes = $true,
    [string]   $GitattributesRef     = 'fddc586cf0f10ec4485028d0d2dd6f73197a4258',
    [string]   $GitignoreRef         = 'dcc0fc7bc2b5ba480cf117ad1be31bafceeaff46',
    [switch]   $Refresh,
    [switch]   $Force
)

Behaviour (summary):

1. Pre-flight: validate cwd is a git repo.
2. Language selection: -Language validated against known set; else interactive multi-select picker with heuristic pre-selection; else abort.
3. Dependency expansion: ASP.NET -> CSharp etc.
4. Compose .gitattributes: header -> Common -> per-lang (stable alpha) -> curated PowerShell block.
5. Compose .gitignore: prefer gibo dump when present and not -Refresh, else assemble from bundled snapshots / network.
6. Header on both files lists pinned SHAs, authority labels, languages, curated additions.
7. Backup-and-replace: if target exists and not -Force, abort; with -Force, copy to <file>.bak (timestamp suffix on collision).
8. Honour -WhatIf.
9. Cache: $env:LOCALAPPDATA\IntelliSDLC.ai\cache\git-defaults\<SHA>\ with tasks/.cache/git-defaults/<SHA>/ fallback.

Pull-SDLC.ai.ps1 Integration

• git rm .gitattributes.template.
• Remove .gitattributes.template from $script:TemplateScaffoldMap and $script:UpstreamManagedPaths.
• Confirm .gitattributes on $script:AlwaysLocalPaths (already there).
• Add .github/templates/git-defaults/ to $script:UpstreamManagedPaths.
• Post-sync hint when missing .gitattributes/.gitignore: print exactly once a one-liner instructing the user to run ./Initialize-GitDefaults.ps1. Do NOT auto-invoke.

Acceptance Criteria

• Invoke-Pester on Initialize-GitDefaults.Tests.ps1 and Pull-SDLC.ai.Tests.ps1 passes 100%.
• PSScriptAnalyzer reports zero new findings vs main on both scripts.
• ./Initialize-GitDefaults.ps1 -Language CSharp,PowerShell,TypeScript -Force in a fresh temp dir composes both files end-to-end, exit 0.
• ./Initialize-GitDefaults.ps1 -Language ASP.NET -Force includes CSharp content.
• .gitattributes.template gone from repo.
• Fresh sync into consumer worktree without .gitattributes prints the hint.