feat: add configurable branch prefix to avoid naming conflicts

[note89]

Problem

When a repository has an existing branch named claude, the action fails with a 422 error when trying to create branches like
claude/issue-123. This is due to Git's limitation where you cannot have both a branch named X and branches named X/....

This can easily happen if:

• You have a team member named Claude who creates their own branch
• You previously created a claude branch for testing (what happend to us, took a long time to figure out)
• You have a feature/module/component named "claude" in your project

Solution

This PR adds a configurable branch_prefix input parameter that allows users to customize the prefix used for branch creation.

Changes

• Add branch_prefix input parameter to action.yml (defaults to 'claude' for backward compatibility)
• Pass branch prefix through context and use it for branch creation
• Improve error serialization to show full error details instead of [Object ...]
• Add detailed error messages for 422 errors explaining the Git naming conflict
• Provide clear solutions when branch creation fails

Example Usage

- uses: anthropics/claude-code-action@v1
  with:
    branch_prefix: 'claude-agent'  # Avoids conflicts with existing 'claude' branch

Testing

Successfully tested on a private repository which had an existing claude branch causing 422 errors. With this fix
 and branch_prefix: 'claude-agent', the action now works correctly.