fix: initialize submodules in new worktrees

[njbrake]

Description

• initialize submodules after creating a new git worktree when the checkout contains a .gitmodules file
• allow local-path submodule URLs for that update step so workspace-local .claude style submodules populate correctly
• add a regression test that creates a real local submodule and verifies its contents appear in the new worktree
• closes Worktrees don't copy submodules #932

Verification:

• cargo test --lib create_worktree_initializes_submodules
• cargo test --lib create_worktree

PR Type

• New Feature
• Bug Fix
• Refactor
• Documentation
• Infrastructure / CI

Checklist

• I understand the code I am submitting
• New and existing tests pass
• Documentation was updated where necessary
• For UI changes: included screenshot or recording

AI Usage

• No AI was used
• AI was used for drafting/refactoring
• This is fully AI-generated

AI Model/Tool used:
Codex (GPT-5)

Any Additional AI Details you would like to share:
Implemented the worktree submodule initialization fix and regression test directly from issue triage for #932.

• I am an AI Agent filling out this form (check box if true)

[Seluj78]

Tested locally, I can confirm my .claude submodule was copied ;)

[Seluj78]

Ah well, sorry I didn't test far enough, I switched to another branch (fix/separate-session-title-and-branch) and when I tried to delete the session I created I got this error (expected since I no longer had the code to delete the submodule)

[njbrake]

Ah well, sorry I didn't test far enough, I switched to another branch (fix/separate-session-title-and-branch) and when I tried to delete the session I created I got this error (expected since I no longer had the code to delete the submodule)

In this case, I think we have a 'force' delete option that should still let you remove the session 🤞

[Seluj78]

yess it worked perfectly for this 😎

[njbrake]

@Seluj78 thank you for all the testing this is suuuuuper helpful 🙏 🚀