fix(standalone): don't add shell config entries on re-run

[Copilot]

The standalone install script appended TYPESPEC_PATH to shell config files (.bashrc, .zshrc, etc.) on every run, creating duplicate entries. Additionally, the --skip-shell / -s flag was silently broken due to a variable name mismatch (SKIP_SHELL vs skip_shell).

Changes

• Auto-detect existing shell config: Before appending to shell config files, each shell branch now checks grep -qF "TYPESPEC_PATH" "$CONF_FILE" and skips if already present, printing an informational message instead
• Fix --skip-shell flag: parse_args() was setting SKIP_SHELL (uppercase) while the exit check read skip_shell (lowercase) — different variables in bash, so the flag had no effect; normalized to skip_shell throughout

• Fixes Standalone script should not add to bashrc when reruning 2nd time #6263

[azure-pipelines]

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[azure-sdk-automation]

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[timotheeguerin]

@copilot run formatter

[Copilot]

@copilot run formatter

Done — ran pnpm format, it normalized 2>/dev/null → 2> /dev/null to match the rest of the file's style. Committed in 2e35e08.