Don't pass duplicate /d flag in Windows cmd use-on-cd hook

[emerson-d-lopes]

Fixes #1556

Problem

The generated cd.cmd always runs cd /d %*. When the user supplies their own /d (e.g. cd /d D:\), the flag is passed twice and cmd fails with:

The syntax of the command is incorrect.

Fix

Add /d only when the first argument isn't already /d:

if /i "%~1" == "/d" cd %*
if /i not "%~1" == "/d" cd /d %*

The shape is deliberate — each alternative fails in a non-obvious way (all verified empirically on Windows 11 cmd.exe):

• No parenthesized if/else blocks: %* may contain ) — cd /d C:\Program Files (x86) inside a ( ... ) block closes the block early and breaks the command. The current single-line form handles it; a block would regress it.
• No setlocal: endlocal reverts directory changes, which would break cd entirely.
• No goto/labels: the file is stored with LF endings and embedded byte-exact via include_bytes!; cmd's label scanning is unreliable with LF-only line endings. The two guarded ifs avoid labels altogether.
• First-argument check only (rather than scanning all args): native cd itself only accepts /d before the path (cd C:\ /d fails with "The system cannot find the path specified"), so positions after the first never work anyway and are passed through to native cd to produce its normal error.

Testing

WinCmd e2e tests are currently skip-listed (e2e/describe.ts), so I verified the behavior directly on Windows 11 with a 15-case matrix against real cmd.exe, comparing old script vs new script vs native cd:

Case	Old	New
cd /d Y:\ (cross-drive via subst, dir contains .node-version)	❌ syntax error	✅ cd works + fnm use --silent-if-unchanged fires
cd /D Y:\ (uppercase)	❌	✅
cd .., cd \, cd (no args)	✅	✅ unchanged
cd "C:\Program Files" / unquoted cd C:\Program Files	✅	✅ unchanged
cd C:\Program Files (x86) and cd /d C:\Program Files (x86)	✅ / ❌	✅ / ✅
nonexistent dir	stays put + native error	unchanged
.node-version / .nvmrc / FNM_VERSION_FILE_STRATEGY=recursive triggers	✅	✅ unchanged (verified with a stubbed fnm on PATH)
environment variable leakage into the session	none	none (fix uses no variables)

Also verified:

• The LF-endings variant of the script (what release binaries embed) passes the same cases.
• cargo test — 28/28 pass, including shell::windows_cmd::tests::use_on_cd_quotes_macro_path.
• Built fnm.exe env --use-on-cd --shell cmd generates the fixed script; re-ran key cases against the generated file.
• Added e2e/repros/issue-1556.test.ts per AGENTS.md (WinCmd-only; it self-skips today but will cover this once WinCmd e2e is enabled). Compiles under jest, prettier clean.
• Changeset included (patch).

🤖 Generated with Claude Code

https://claude.ai/code/session_01U5TQ1y3yBEDx73tYTHoUBj