git: stop overwriting the user's identity on every run

[presto8]

The tracked gitconfig hard-coded a [user] section:

[user]
    email = unknown@unknown.com
    name  = Unknown User

setup_git copies that file over ~/.gitconfig on every run, so every
provisioned machine silently had its git identity replaced by the
placeholder. Commits then get authored as 'Unknown User
unknown@unknown.com' with no error and no prompt -- git only refuses
to commit when an identity is absent, not when it is wrong.

Drop the [user] section from the shipped file, and have setup_git carry
the identity across the overwrite:

• read user.name/user.email before the copy and restore them after
• treat the old placeholder values as absent, so machines already
  clobbered by a previous run recover instead of keeping them
• let GIT_USER_NAME / GIT_USER_EMAIL override (these can be exported,
  or set in config.local once that mechanism lands)
• print the two commands to run when no identity can be determined,
  rather than leaving a config that looks populated but is wrong

Verified against throwaway HOMEs: a real identity is preserved; the
placeholder is discarded; a machine with no identity gets the hint; an
explicit GIT_USER_* wins over an existing value; and the aliases from
the shipped gitconfig are still installed in every case.

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🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The changes directly address the identity-clobbering behavior with a small, well-contained update that preserves existing settings and avoids reintroducing placeholder values.

Pull request overview

This PR prevents setup_git from clobbering a user’s Git author identity by removing the hard-coded [user] placeholder from the shipped gitconfig and explicitly preserving/restoring user.name / user.email around the ~/.gitconfig overwrite.

Changes:

• Capture existing user.name / user.email before overwriting ~/.gitconfig, treating the prior placeholder values as “absent”.
• Allow GIT_USER_NAME / GIT_USER_EMAIL to override the preserved values, then re-apply them via git config --global.
• Remove the [user] section from the shipped gitconfig and document the identity behavior inline.

File summaries

File	Description
setup	Preserves/restores Git identity around overwriting ~/.gitconfig, supports env overrides, and prints guidance when identity is missing.
gitconfig	Removes hard-coded placeholder identity and documents why the file intentionally lacks a [user] section.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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