feat(Pull-SDLC): replace -SetupGitHubSsh with -SetupGit

[MarkMichaelis]

Summary

Replaces the misguided -SetupGitHubSsh switch (added in #164, partially walked back in #170) with a focused -SetupGit that does what a per-repo sync script should actually do: scaffold repo-level git files. No more touching SSH keys, ssh-agent, machine-wide git settings, the gh CLI, or URL rewrites -- protocol choices stay with the user.

Per the discussion on #169, the old switch was wrong on two axes:

1. Protocol is not the script's business. Whether a user connects to GitHub over HTTPS, SSH-22, or SSH-443 is a per-machine choice.
2. GitHub-specific framing was too narrow. The pieces a fresh repo actually wants pre-configured are .gitignore and .gitattributes -- git configuration, not GitHub configuration. Hence -SetupGit, not -SetupGitHub.

What -SetupGit does

1. .gitattributes: creates the file with a minimal line-ending normalization baseline (* text=auto eol=lf) when missing. Never overwrites an existing file.
2. .gitignore: union-merges upstream entries into the consumer copy via the existing Merge-FileFromUpstream code path (the same one the normal sync uses).

Every step prints [skip] or [add] so the user sees exactly what changed.

Removed (breaking)

-SetupGitHubSsh was recent (#164) and the broader direction was already being walked back. Clean removal beats a deprecation alias:

• -SetupGitHubSsh and -SkipKeyUpload parameters
• Invoke-SetupGitHubSsh function
• Helpers: Get-GitHubSshKeyPath, Test-GitHubSshAgentRunning, Test-GhInstalled, Get-GitConfigAllValues, Get-GhGitProtocol, Add-GitConfigValueIfMissing
• 6 obsolete Pester Describe blocks covering the removed helpers

Tests added

• Invoke-SetupGit behavior (5 tests): creates .gitattributes with the expected baseline when missing, leaves an existing .gitattributes alone, forwards the upstream ref correctly to Merge-FileFromUpstream, does not invoke ssh-keygen/gh/git config --global (contract guard), is idempotent.
• Removal regression tests (4 tests) asserting the script no longer declares -SetupGitHubSsh/-SkipKeyUpload, no longer defines Invoke-SetupGitHubSsh, and contains no code references to git config --global, gh config set, ssh-keygen, or url.insteadOf. These are the real ratchet -- they fail loudly if anyone later re-adds protocol-level mutations.

Diff stats

2 files changed, +155 / -362. Net deletion of ~200 lines.

Stacked on

This PR is stacked on #170 (which removes the unsolicited drift nudge). Merge #170 first, then this PR will rebase cleanly onto main.

Verification

\\powershell
Invoke-Pester -Path .\Pull-SDLC.ai.Tests.ps1 -FullNameFilter '-SetupGit','Invoke-SetupGit','-SetupGitHubSsh removed','post-sync output'
\\

Local result: 10 passed, 0 failed. Broader subset (31 tests across ConvertTo-GitHubRepoSlug, Resolve-OpenSyncPRAction, Test-IsCiEnvironment, Test-IsUpstreamRepo, plus all new tests): 31 passed, 0 failed.

Closes #171

[MarkMichaelis]

Closing in favor of #161 (issue #160), which was opened ~4 hours before this PR and tackles the same scope with a substantially more thoughtful design:

• Separate Initialize-GitDefaults.ps1 script rather than a switch on the sync script.
• Language-aware composition (C#, PowerShell, TypeScript, ASP.NET).
• Pinned community templates from alexkaratarakis/gitattributes and github/gitignore.
• Bundled offline snapshots with pinned SHAs and SOURCES.md.
• gibo ecosystem integration.

This PR was built in a session that didn't know about #161. Apologies for the duplicate work. The minimal * text=auto eol=lf baseline approach here doesn't add anything that the language-aware composition in #161 won't already cover.

The orthogonal "remove unsolicited drift nudge" fix is preserved in #170 (parent of this PR's branch) and will land independently.

Closes nothing -- issue #171 is being closed separately as superseded by #160.