feat: Add support for creating instances from main branch

[matthew-petty]

Adds the ability to create claude-squad instances from the main/master branch instead of the current HEAD. This is useful when you want to start fresh work from a stable base rather than your current working branch.

Problem

When working on a feature branch, users currently can only create new instances from their current HEAD. This means if you're midway through changes and want to start a separate task, the new instance inherits your work-in-progress state rather than starting from a clean main branch.

Solution

• Add keyboard shortcuts m and M to create instances from the repository's main/master branch
• Auto-detect the default branch (checks origin/HEAD, then main, then master)
• Display visual indicators to show instance creation source: [main] prefix for main-based, * prefix for HEAD-based
• Add optional DefaultBranch config field for repos with custom default branches

Changes

• New keybindings: m (new from main), M (new with prompt from main)
• Visual indicators: Clear prefixes showing instance creation source
• Smart branch detection: Automatically finds the repository's default branch
• Backwards compatible: Existing n and N shortcuts unchanged

Testing

• Added unit tests for GetDefaultBranch() function
• Added tests for BaseRef field serialization/deserialization
• Verified all existing tests pass
• Manual testing with various repository configurations

---

Example of the new UI indicators in the session list:

* my-feature-instance     (created from HEAD)
[main] stable-work        (created from main branch)

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[matthew-petty]

I have read the CLA Document and I hereby sign the CLA

[matthew-petty]

This PR implements the TODO that was added in commit a287824:

File: session/git/worktree_ops.go
Line: 112 (on main branch)
TODO: // TODO: we might want to give an option to use main/master instead of the current branch.
Author: @mufeez-amjad
Date: March 23, 2025
Commit: a287824