PowerShell hygiene burndown: kill git -C, sanitize $Comment, backport retry-with-backoff

[PolyphonyRequiem]

Context

Three PowerShell hygiene issues surfaced by Liszt (PowerShell) during the 2026-05-28 fan-out, all in or adjacent to the production launcher and worktree manager:

1. git -C in production launcher — scripts/Invoke-PolyphonySdlc.ps1:833 is the last surviving git -C in production code. On Windows, git -C with paths containing spaces produces unreliable results and can silently misdetect the platform (falling back to 'ado' without warning, or embedding a multi-line error string into $remoteUrl). Fix: Push-Location/Pop-Location wrapper.

2. $Comment injection surface — scripts/Invoke-PolyphonySdlc.ps1:357,424-456 embeds $Comment verbatim into a PowerShell here-string that is passed to pwsh -Command. If $Comment contains newlines or backticks (reachable via conductor workflow inputs to the reset path), the child window opens with a syntax error and the exit sidecar records "exit_code": null with no diagnostic. Fix: sanitize $Comment at parameter-bind time (reject or strip [\r\n + backtick]).

3. Missing retry-with-backoff in worktree teardown — .conductor/registry/scripts/worktree-manager.ps1:254-272 deletes git worktrees without the retry-with-backoff pattern that was backported to src/Polyphony/Journal/Reset/Deleters/GitWorktreeDeleter.cs:37-64. Worktree deletion on Windows is prone to transient file-lock failures; without retry, the script fails hard. Fix: backport the C# retry pattern to the PS script.

Parent epic: #522 (Squad short-term wins, 2026-05-28 fan-out)

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Acceptance criteria

• scripts/Invoke-PolyphonySdlc.ps1:833: git -C $mainWorktree remote get-url origin replaced with Push-Location $mainWorktree; try { git remote get-url origin } finally { Pop-Location } pattern
• scripts/Invoke-PolyphonySdlc.ps1:357,424-456: $Comment validated/sanitized against newlines and backtick injection before heredoc embedding
• .conductor/registry/scripts/worktree-manager.ps1:254-272: retry-with-backoff loop backported from GitWorktreeDeleter.cs:37-64 for worktree deletion

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Owner

• Primary: Liszt (PowerShell — production launcher + worktree-manager)
• Secondary: Reich (worktree teardown retry — owns git/worktree seam)

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Estimated effort

4–6 hours

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File anchors

• scripts/Invoke-PolyphonySdlc.ps1 (lines 833, 357, 424-456)
• .conductor/registry/scripts/worktree-manager.ps1 (lines 254-272)
• src/Polyphony/Journal/Reset/Deleters/GitWorktreeDeleter.cs (lines 37-64 — retry source of truth)

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In-situ clause

Apply DU patterns and hygiene fixes opportunistically where natural in your scope. Don't make these the primary focus.