bug: bootstrap overwrites existing files despite "never overwrites" guarantee

[jwm4]

Bug Description

The user guide (docs/user-guide.md, line 101) states that bootstrap is "Safe by Design" and "Never overwrites existing files." However, only CONTRIBUTING.md and CODE_OF_CONDUCT.md have existence guards. All other generated files are silently overwritten if they already exist:

• .github/workflows/agentready-assessment.yml
• .github/workflows/tests.yml
• .github/workflows/security.yml
• .github/workflows/repomix-update.yml
• .github/ISSUE_TEMPLATE/bug_report.md
• .github/ISSUE_TEMPLATE/feature_request.md
• .github/PULL_REQUEST_TEMPLATE.md
• .github/CODEOWNERS
• .pre-commit-config.yaml
• .github/dependabot.yml

Steps to Reproduce

1. Create a repo with a customized .pre-commit-config.yaml or CODEOWNERS
2. Run agentready bootstrap .
3. Observe that the existing file is replaced with the template version

Expected Behavior

Bootstrap should skip any file that already exists, matching the documented guarantee. Ideally it should log a message like "Skipping .pre-commit-config.yaml (already exists)" so the user knows.

Relevant Code

src/agentready/services/bootstrap.py lines 180-189: _write_file() unconditionally opens the file with "w" mode. Only _generate_docs() (lines 166, 173) checks for existing files before writing.

Acceptance Criteria

• _write_file() or each caller checks for file existence before writing
• Existing files are never overwritten
• Skipped files are reported in the output (e.g., "Skipping .pre-commit-config.yaml (already exists)")
• --dry-run output distinguishes between "would create" and "would skip (exists)"
• Unit tests verify that existing files are preserved

Submitted by Bill Murdock with assistance from Claude Code.