#7494: feat(completion): PowerShell automated install & uninstall

Splits out the PowerShell automated install & uninstall work into its own PR. This branch applies the cleaned/squashed commit from the previous rework and contains: - docs/cli/completion.md - scripts/postinstall.js - CLI wiring for PowerShell completion - completion installer tests Tests for the completion installer pass locally (`src/completion-install.test.ts`, `src/postinstall-completion.test.ts`). Notes & concerns: - A Greptile review flagged sanitization behavior mismatch (sanitization reported but not applied) and brittle PowerShell profile targeting/uninstall logic — please pay attention to `src/agents/session-tool-result-guard.ts` and the completions uninstall flow. - Backup branch with original commits: `backup/feat/pwsh-completion-install-uninstall-before-reset`. If you prefer, I can split out the sanitization changes for separate review. /cc @steipete