fix(cli): tolerate safe leftovers in smpy new --dest . (#148)

[antosubash]

Summary

• smpy new --dest . was refusing any non-empty destination, which blew up the common gh repo create myapp && cd myapp && smpy new myapp --dest . flow the moment .git/, .gitignore, or README.md was present (closes smpy new --dest . refuses non-empty directories — blocks scaffolding into a fresh git init / worktree #148).
• Scaffolding now tolerates a small allowlist (SAFE_PRESERVED_NAMES in scaffolding.py) at the target — VCS metadata (.git/, .gitignore, .gitattributes), docs (README*, LICENSE*, COPYING, CHANGELOG.md, CONTRIBUTING.md, CODE_OF_CONDUCT.md), and editor/tooling metadata (.claude/, .vscode/, .idea/, .editorconfig, .DS_Store). Any other pre-existing entry still raises FileExistsError, naming the colliding files so the user knows what to move aside.
• When the scaffold would write over a tolerated pre-existing file (e.g. user has a .gitignore, scaffold also has one), the scaffold's copy is skipped — the user's file wins. smpy new surfaces a Preserved existing files notice listing what was skipped so users can merge in scaffold content by hand if they care.
• create_workspace / create_host accept a preserve_existing set and return the list of preserved paths; _require_empty_dest accepts the same set; create_app_project returns (host_dir, preserved).

Test plan

• uv run pytest framework/cli/tests/ — 121 passed (3 new regression tests in test_cli_new_dest_tolerance.py)
• uv run ruff check framework/cli/ + uv run ruff format --check framework/cli/ clean
• uv run ty check framework/cli/simple_module_cli/ clean
• uv run python scripts/check_file_size.py — no files exceed 300 lines
• Manual smoke: smpy new myapp --yes --no-install --dest . against a directory with .git/, .gitignore (*.pyc), README.md, and LICENSE succeeds; user files are byte-identical after the run; scaffold's Makefile, pyproject.toml, host/, modules/ land alongside them; CLI prints the preservation notice for .gitignore and README.md.
• Manual smoke: smpy new myapp --yes --no-install --dest . against a directory with notes.txt still hard-fails (exit=1), names notes.txt in the error, and leaves the file untouched.

https://claude.ai/code/session_01K84RjsX1ToXorMyuaxNBT6

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