Bug Description

When a repository's .git/config enables index.skipHash indirectly via Git's
feature.manyFiles = true (or feature.experimental = true) meta-setting,
go-git fails to read the index with:

index decoder: invalid checksum

Git writes the index trailer as 20 (or 32 for SHA-256) zero bytes when
index.skipHash is in effect. go-git supports index.skipHash when the key is
present literally under [index], but it does not expand Git's
feature.* aliases, so Config.Index.SkipHash stays unset and the decoder
verifies the checksum against the all-zero trailer and rejects the index.

go-git Version

v6.0.0-alpha

Steps to Reproduce

git init repo && cd repo
git config feature.manyFiles true   # implies index.skipHash=true in Git >= 2.40
echo hi > a.txt && git add a.txt && git commit -m init

package main

import (
    "fmt"

    git "github.com/go-git/go-git/v6"
)

func main() {
    repo, err := git.PlainOpen("repo")
    if err != nil {
        panic(err)
    }
    wt, err := repo.Worktree()
    if err != nil {
        panic(err)
    }
    _, err = wt.Status()
    fmt.Println("status err:", err) // => index decoder: invalid checksum
}

git status on the same repo works fine. Adding git config index.skipHash true
explicitly also makes go-git succeed, confirming the gap is alias expansion, not
checksum handling.

Additional Information

Expected behavior

go-git should honor Git's feature.manyFiles / feature.experimental
meta-settings, which expand to (per Git docs):

• feature.manyFiles=true → index.skipHash=true, index.version=4, core.untrackedCache=true
• feature.experimental=true → index.skipHash=true (among others)

so that reading such an index does not error. At minimum, index.skipHash
should be inferred from feature.manyFiles / feature.experimental when the
literal index.skipHash key is absent.

Notes

• feature.manyFiles is increasingly common: some Git GUIs/tooling and large
  monorepos enable it for performance.
• A repo with this config also typically carries UNTR (untracked cache) and
  FSMN (fsmonitor) index extensions; those decode fine as optional extensions —
  the only failure is the zero-checksum verification.
• Possibly related: Bug: worktree cannot be read if created with version 4 #1123 (index version 4 read failure).