Introduce worktree helper scripts for faster multi-branch development in PowerToys (#42076)

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## Summary of the Pull Request
This pull request introduces a new suite of helper scripts for managing
Git worktrees in the `tools/build` directory, along with comprehensive
documentation. The scripts streamline common workflows such as creating,
reusing, and deleting worktrees for feature branches, forks, and
issue-based development, making it easier for developers to work on
multiple changes in parallel without duplicating the repository. Each
script is provided as both a PowerShell (`.ps1`) and Windows batch
(`.cmd`) wrapper for convenience. A detailed markdown guide explains
usage patterns, scenarios, and best practices.

**New worktree management scripts:**

* Added `New-WorktreeFromFork.ps1`/`.cmd` to create a worktree from a
branch in a personal fork, handling remote creation and branch tracking
automatically.
[[1]](diffhunk://#diff-ea4d43777029cdde7fb9fda8ee6a0ed3dcfd75b22ed6ae566c6a77797c8bef54R1-R111)
[[2]](diffhunk://#diff-1314b08f84ac8c2e7d020e5584d9f2f19dbf116bbc13c14de0de432006912cfeR1-R4)
* Added `New-WorktreeFromBranch.ps1`/`.cmd` to create or reuse a
worktree for an existing local or remote branch, with logic to fetch and
track branches as needed.
[[1]](diffhunk://#diff-07c08acfb570e1b54647370cae17e663e76ee8cb09614cac7a23a9367f625a3eR1-R69)
[[2]](diffhunk://#diff-6297be534792c3e6d1bc377b84bcd20b2eb5b3de84d4376a2592b25fc9a88a88R1-R4)
* Added `New-WorktreeFromIssue.ps1`/`.cmd` to create a new issue branch
from a base ref (default `origin/main`), slugifying the issue title for
branch naming.
[[1]](diffhunk://#diff-36cb35f3b814759c60f770fc9cc1cc9fa10ceee53811d95a85881d8e69c1ab07R1-R67)
[[2]](diffhunk://#diff-890880241ffc24b5d29ddb69ce4c19697a2fce6be6861d0a24d02ebf65b35694R1-R4)
* Added `Delete-Worktree.ps1`/`.cmd` to safely remove a worktree, with
options to force removal, keep the local branch, or retain orphan fork
remotes. Includes robust error handling and manual recovery guidance.
[[1]](diffhunk://#diff-8a335544864c1630d7f9bec6f4113c10d84b8e26054996735da41516ad93e173R1-R120)
[[2]](diffhunk://#diff-19a810e57f8b82e1dc2476f35d051eb43f2d31e4f68ca7c011c89fd297718020R1-R4)

**Documentation:**

* Introduced `Wokrtree-Guidelines.md`, a comprehensive guide covering
the purpose, usage, flows, naming conventions, troubleshooting, and best
practices for the new worktree scripts.

.github/actions/spell-check/expect.txt

@@ -580,6 +580,7 @@ GETSCREENSAVERRUNNING
 GETSECKEY
 GETSTICKYKEYS
 GETTEXTLENGTH
+gitmodules
 GHND
 GMEM
 GNumber
@@ -1327,6 +1328,7 @@ PRTL
 prvpane
 psapi
 pscid
+pscustomobject
 PSECURITY
 psfgao
 psfi
@@ -1977,6 +1979,7 @@ WORKSPACESEDITOR
 WORKSPACESLAUNCHER
 WORKSPACESSNAPSHOTTOOL
 WORKSPACESWINDOWARRANGER
+Worktree
 wox
 wparam
 wpf