feat: support CLAUDE_SQUAD_HOME environment variable for custom config directory

[zimathon]

Summary

Add support for the CLAUDE_SQUAD_HOME environment variable to allow users to customize the configuration directory location.

Closes #245

Changes

• Add EnvConfigDir constant for the environment variable name
• Modify GetConfigDir() to check for the environment variable first
• Support tilde expansion (~/) in the environment variable value
• Add comprehensive tests for the new functionality

Use Cases

1. Per-repository session isolation:

#!/bin/bash
REPO_ROOT=$(git rev-parse --show-toplevel 2>/dev/null)
REPO_NAME=$(basename "$REPO_ROOT")
export CLAUDE_SQUAD_HOME="$HOME/.claude-squad/repos/$REPO_NAME"
exec claude-squad "$@"

2. Project-specific configurations:

export CLAUDE_SQUAD_HOME="$PWD/.claude-squad"

3. Testing/development:

CLAUDE_SQUAD_HOME=/tmp/cs-test claude-squad

Benefits

• Backwards compatible: Default behavior unchanged when env var is not set
• Simple implementation: Minimal code change required
• Flexible: Users can implement their own isolation strategies
• Reduces state corruption impact: Issues in one config dir don't affect others

Test plan

• go build . succeeds
• go test ./config/... passes
• Manual verification with CLAUDE_SQUAD_HOME=/tmp/test-cs ./claude-squad debug
• Tilde expansion works correctly

Verification

$ CLAUDE_SQUAD_HOME=/tmp/test-cs ./claude-squad debug
Config: /tmp/test-cs/config.json
{
  "default_program": "/opt/homebrew/bin/claude",
  "auto_yes": false,
  "daemon_poll_interval": 1000,
  "branch_prefix": "y.sasajima/"
}

[github-actions]

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

[zimathon]

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