[Bug]: Test-FeatureJsonMatchesFeatureDir was missing in common.ps1 — this was a missing function that caused setup-tasks.ps1 to fail.

### Bug Description

On start of `/speckit.tasks` in PowerShell environment, it attempts to call function
`Test-FeatureJsonMatchesFeatureDir`
An error is thrown and there is no such commandlet.

CoPilot eventually worked around the problem.

```shell
.\.specify\scripts\powershell\setup-tasks.ps1 -Json
C:\Code\private\app\.specify\scripts\powershell\setup-tasks.ps1 : The term
'Test-FeatureJsonMatchesFeatureDir' is not recognized as the name of a cmdlet, function, script file, or
operable program. Check the spelling of the name, or if a path was included, verify that the path is
correct and try again.
At line:1 char:1
+ .\.specify\scripts\powershell\setup-tasks.ps1 -Json
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : ObjectNotFound: (Test-FeatureJsonMatchesFeatureDir:String) [setup-tasks.ps1]
, CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException,setup-tasks.ps1
```

### Steps to Reproduce

1: Run `/speckit.tasks`
2: Errors noted for missing function `Test-FeatureJsonMatchesFeatureDir`

The function checks if feature.json pins an existing feature directory, and if it does, branch naming validation gets skipped.

Since the function is missing from common.ps1, this validation logic can't run.

### Expected Behavior

No error should have been reported.

### Actual Behavior

```shell
.\.specify\scripts\powershell\setup-tasks.ps1 -Json
C:\Code\private\app\.specify\scripts\powershell\setup-tasks.ps1 : The term
'Test-FeatureJsonMatchesFeatureDir' is not recognized as the name of a cmdlet, function, script file, or
operable program. Check the spelling of the name, or if a path was included, verify that the path is
correct and try again.
At line:1 char:1
+ .\.specify\scripts\powershell\setup-tasks.ps1 -Json
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : ObjectNotFound: (Test-FeatureJsonMatchesFeatureDir:String) [setup-tasks.ps1]
, CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException,setup-tasks.ps1
```

Since the function is missing from common.ps1, this validation logic can't run.

CoPilot eventually determined what the function should have returned and worked around the problem.

### Specify CLI Version

0.8.13

### AI Agent

GitHub Copilot

### Operating System

Windows 11 (10.0.26220)

### Python Version

Python 3.12.6

### Error Logs

```shell
.\.specify\scripts\powershell\setup-tasks.ps1 -Json
C:\Code\private\app\.specify\scripts\powershell\setup-tasks.ps1 : At
C:\Code\private\app\.specify\scripts\powershell\common.ps1:141 char:122
+ ... -Path $RepoRoot $pinnedRelative) -ErrorAction SilentlyContinue)?.Path
+ ~~~~~~
Unexpected token '?.Path' in expression or statement.
At C:\Code\private\app\.specify\scripts\powershell\common.ps1:142 char:102
+ ... h -LiteralPath $ActiveFeatureDir -ErrorAction SilentlyContinue)?.Path
+ ~~~~~~
Unexpected token '?.Path' in expression or statement.
At line:1 char:1
+ .\.specify\scripts\powershell\setup-tasks.ps1 -Json
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : ParserError: (:) [setup-tasks.ps1], ParseException
+ FullyQualifiedErrorId : UnexpectedToken,setup-tasks.ps1
```

### Additional Context

This speckit was in-place upgraded from a previous version ~ 1 month old

.specify/scripts/powershell/common.ps1 と .specify/scripts/powershell/setup-tasks.ps1 からの呼び出しから始めます。PowerShell で setup-tasks.ps1 -Json を実行して失敗を再現し、その後 Test-FeatureJsonMatchesFeatureDir が利用可能であり、機能ディレクトリの検証が報告されたコマンドエラーまたはパーサーエラーなしで完了することを確認します。

索引モデルが issue の本文から書いたものです。