Add todo-tree extension using Zed tasks

[alexandretrotel]

This PR reintroduces the todo-tree extension using Zed tasks instead of agent slash commands.

Context:

• The previous PR (Add todo-tree slash command extension #4110) was closed because slash commands are intended only for agents.
• This implementation follows the recommendation to use tasks instead.

Changes:

• Removed all slash-command usage
• Implemented todo-tree functionality via Zed tasks and guides for users to do it
• Extension remains non-agent-based and aligns with current platform expectations

Happy to adjust based on feedback (again)!

[Copilot]

Pull request overview

This PR adds the todo-tree extension to the Zed extensions registry using Zed tasks instead of agent slash commands, following feedback from a previous closed PR (#4110).

• Adds todo-tree as a new submodule pointing to the extension repository
• Registers the extension in extensions.toml with version 0.3.0
• Configures git submodule references in .gitmodules

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
extensions/todo-tree	Adds the todo-tree extension submodule at commit 7ccedaa
extensions.toml	Registers todo-tree extension with version 0.3.0
.gitmodules	Configures git submodule for todo-tree pointing to the GitHub repository

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