Description

The branch name validation in src/github/operations/branch.ts rejects branch names containing @, even though @ is a valid character in git branch names. This causes the action to fail on PRs with @ in the branch name.

Steps to Reproduce

1. Create a PR from a branch with @ in the name, e.g. TICKET-123@add-feature
2. The Claude PR review workflow triggers
3. The prepare step fails with:

   Prepare step failed with error: Invalid branch name: "TICKET-123@add-feature". 
   Branch names must start with an alphanumeric character and contain only alphanumeric characters, 
   forward slashes, hyphens, underscores, or periods.
   

Root Cause

The allowlist regex in src/github/operations/branch.ts:

const validPattern = /^[a-zA-Z0-9][a-zA-Z0-9/_.-]*$/;

Does not include @ in the allowed character set. While @{ is a special git reflog syntax (and is correctly rejected separately), a standalone @ or @ followed by non-{ characters is perfectly valid in git branch names.

Expected Behavior

Branch names containing @ (but not @{) should be accepted. The regex could be updated to:

const validPattern = /^[a-zA-Z0-9][a-zA-Z0-9/_.@-]*$/;

With the existing @{ check still catching the unsafe git reflog syntax.

Environment

• Action version: v1.0.27
• Event: pull_request

Impact

Any PR with @ in the branch name will fail the Claude review workflow. This is a common pattern in some teams (e.g. TICKET@description convention).