Add initial PowerShell OSS bounty audit notes and local repro fixtures

[kajf]

Motivation

• Start a structured, reproducible audit for the Microsoft OSS bounty program targeting PowerShell/PowerShell by documenting scope, environment, and first-pass triage findings.
• Capture setup steps and environment constraints to make subsequent research and reporting reproducible.
• Provide small, local-only repros for module-path, temporary-file, and remoting-proxy temp-path observations to aid deeper follow-up audits.

Description

• Add research/powershell-oss-bounty-initial-audit.md with scope, setup commands, attack-surface map, ranked code-paths, dead ends, and next steps.
• Add three local repro/triage fixtures under research/repros/: module-path-shadowing.ps1, new-temporary-file-permissions.ps1, and remoting-proxy-temp-path-shape.ps1.
• Add .gitignore entry to exclude the local upstream/ clone used for builds and research artifacts.
• Commit these files and open a PR titled "Add initial PowerShell OSS bounty audit notes" to record the audit baseline.

Testing

• Successfully built the core engine with Start-PSBuild -UseNuGetOrg -NoPSModuleRestore, producing pwsh at src/powershell-unix/.../publish/pwsh. (succeeded)
• Executed the module-path fixture with the locally-built pwsh using research/repros/module-path-shadowing.ps1 and observed expected PSModulePath precedence behavior. (succeeded)
• Executed the temp-file fixture with the locally-built pwsh using research/repros/new-temporary-file-permissions.ps1 and confirmed Unix mode 600 for created temp files. (succeeded)
• Executed the remoting-proxy triage helper research/repros/remoting-proxy-temp-path-shape.ps1 to record observations about RemoteDiscoveryHelper.GetModulePath behavior. (succeeded)
• Attempted a full/default build and a focused Start-PSPester run but both were blocked by environment proxy failures when restoring PowerShell Gallery dependencies (HTTP 403) so those flows did not complete. (blocked)

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Codex Task