feat: add an actionable optional-import guard with install hints

[hasansezertasan]

Summary

• Adds core/_imports.require(module, *, feature, extra=None): imports an optional/lazy dependency or raises a ModuleNotFoundError naming the actually-missing module (via the original exception's name) and the exact pip install command, instead of a bare traceback.
• Wires it into the CLI root's lazy-imported, non-primary component subcommands (interactive/web/mcp/worker, both the Typer and argparse variants, plus the minimal launcher's default callback) and the corresponding component of the default() callback used when include_cli is off. The GUI subcommand is left unguarded since it only depends on stdlib tkinter.
• Documents the helper in docs/template-architecture.md under the core/ package.

Closes #172

Test plan

• Regenerated a project with preset=full (copier copy --data-file .example-input.yml --data preset=full --defaults --trust . /tmp/test-172)
• uv run --locked tox run -e style passes (ruff, mypy, basedpyright, ty, pyrefly, zuban, pylint, slotscheck, import-linter, taplo, typos, editorconfig-checker, sphinx-lint, deadfixtures)
• uv run --locked tox run passes across all Python envs
• coverage report shows 100% (gate is fail_under = 99)

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📥 Commits

Reviewing files that changed from the base of the PR and between 825f420 and d8d5bea.

📒 Files selected for processing (6)

• docs/template-architecture.md
• template/src/{{github_repo_name}}/core/_imports.py.jinja
• template/src/{{github_repo_name}}/{% if include_console_root %}cli{% endif %}/app.py.jinja
• template/src/{{github_repo_name}}/{% if include_tui %}tui{% endif %}/app.py.jinja
• template/src/{{github_repo_name}}/{% if include_web %}web{% endif %}/app.py.jinja
• template/tests/core/test_imports.py.jinja

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