feat: add global Engram auto-initialization

[sundaram2021]

Description

Adds opt-in global Engram setup so new git repositories can be bootstrapped automatically from the user's global Engram credentials.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #311

Changes Made

• Added global auto-initialization config stored in ~/.engram/config.json
• Added engram install --auto-init and engram bootstrap --auto
• Added tests and README usage notes for the new bootstrap flow

Testing

• Unit tests pass (python -m pytest tests\test_cli_install.py tests\test_workspace.py)
• Lint passes (python -m ruff check src\engram\cli.py src\engram\workspace.py tests\test_cli_install.py tests\test_workspace.py README.md)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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