Add CI workflows to test install scripts and support local binary override

[svarlamov]

Motivation

• Ensure the installer scripts (install.sh / install.ps1) are exercised automatically across supported OSes and shells.
• Validate that Claude Code hooks and shell PATH changes are installed correctly by the scripts.
• Allow PRs to test against the repository's freshly built git-ai binary instead of the latest release to catch regressions in the local build + install script interaction.

Description

• Add support for a local-binary override by honoring GIT_AI_LOCAL_BINARY in install.sh and install.ps1, copying the provided binary instead of downloading a release.
• Add .github/workflows/install-scripts-local.yml which builds git-ai from source and runs install.sh/install.ps1 in CI (Ubuntu, macOS, Windows), prepares a fake HOME and a fake claude binary, and verifies shell config updates and Claude hooks.
• Add .github/workflows/install-scripts-nightly.yml which runs the public install scripts nightly (and on-demand) across Ubuntu/macOS/Windows, prepares a fake environment for Claude detection, and verifies PATH and hook installation.
• Ensure CI steps set HOME/USERPROFILE via GITHUB_ENV and add fake bin directories to GITHUB_PATH so the installer detects the fake claude binary and writes ~/.claude/settings.json.

Testing

• No automated tests were executed in this change; the new GitHub Actions workflows are added and will run on PR/push (local-build workflow) and on schedule (nightly workflow).
• The local workflow is configured to run cargo build --release --bin git-ai and then run the install scripts with GIT_AI_LOCAL_BINARY set, and the nightly workflow is configured to exercise the public install endpoints; both will report results when CI runs.

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Codex Task

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 5 additional findings in Devin Review.

[devin-ai-integration]

🟡 install.ps1 prints misleading "Downloading" message when using a local binary override

When GIT_AI_LOCAL_BINARY is set, the install.ps1 script unconditionally prints "Downloading git-ai (release: local)..." at line 299, even though no download occurs — the binary is copied locally. The install.sh script correctly handles this with a conditional message ("Using local git-ai binary" vs "Downloading git-ai" at lines 246/252).

Root Cause

The Write-Host at install.ps1:299 was not updated to be conditional when the local binary override was added. In install.sh, the message was properly split:

if [ -n "${GIT_AI_LOCAL_BINARY:-}" ]; then
    echo "Using local git-ai binary (release: ${RELEASE_TAG})..."
    ...
else
    echo "Downloading git-ai (release: ${RELEASE_TAG})..."
    ...
fi

But in install.ps1, the message at line 299 is always printed before the conditional logic at line 316:

Write-Host ("Downloading git-ai (release: {0})..." -f $releaseTag)  # line 299 - always says "Downloading"
...
if (-not [string]::IsNullOrWhiteSpace($env:GIT_AI_LOCAL_BINARY)) {  # line 316
    Copy-Item ...  # actually copies, not downloads
}

Impact: Users and CI logs will show a misleading "Downloading" message when the local binary override is in use, causing confusion during debugging.

(Refers to line 299)

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[devin-ai-integration]

Devin Review found 1 new potential issue.

View 10 additional findings in Devin Review.