feat(create): add --describe with generated-project layout hints

[popwatt]

Summary

From-scratch implementation of the template description mode for agentseek create (issue #90).

• agentseek create bub/default --describe prints the template spec, its description from templates/index.json, the template path, every Cookiecutter input variable from cookiecutter.json, and the project layout the template will generate (top-level entries expanded one level).
• Bundled templates are resolved exactly like normal create (same catalog, same --checkout/--template-repo handling).
• Does not run Cookiecutter and creates no files in describe mode.
• External specs (URL / absolute path) with --describe exit 2 with a clear message.
• Preserves existing --list-templates, --template, --checkout, --no-input, and --filter behavior.

Note: #115 already merged a baseline --describe. This PR is a re-implementation that additionally covers the "key generated-project hints when available" requirement from #90 (the Generated project layout block), which #115 does not print.

Testing

• uv run python -m pytest tests/cli_commands/test_create.py -q (119 passed, 6 skipped)
• uv run python -m pytest tests/ -q (1098 passed, 53 skipped)
• uv run ruff check src/agentseek/cli/commands/create.py tests/cli_commands/test_create.py
• uv run ruff format --check src/agentseek/cli/commands/create.py tests/cli_commands/test_create.py
• Manual: describe output shows Template / Description / Path / Cookiecutter variables / Generated project layout

Refs #90

[popwatt]

#90

[webup]

Thanks for adding the generated-project layout hints. There is one blocking correctness issue:

• _template_layout_hints() enumerates every entry directly under template_dir, but Cookiecutter selects the templated child directory (for example, {{cookiecutter.project_slug}}/) and renders only that tree. As a result, bundled templates such as bub/default report the template repository’s outer, author-facing README.md as generated output even though Cookiecutter ignores it; the output can show a second root-level README.md that will not exist in the generated project.

Please locate the actual project template directory using Cookiecutter’s selection semantics (or equivalent), derive the hints from that directory, and add a regression test asserting that a template-root metadata file is excluded from the generated layout.

[popwatt]

Thanks for the review! Fixed the blocking issue.

_template_layout_hints now locates the templated project directory using Cookiecutter's selection semantics (see cookiecutter.find.find_template: the child whose name contains cookiecutter plus the {{ / }} Jinja2 delimiters) and derives the layout from that directory only. Template-root metadata such as the author-facing README.md, cookiecutter.json, and hooks/ are no longer reported as generated output.

Added regression coverage:

• test_template_layout_hints_uses_cookiecutter_project_dir — asserts a template-root README.md is excluded from the layout and that hints come from {{cookiecutter.project_slug}}/.
• test_template_layout_hints_empty_without_templated_dir — a template without a templated project dir yields no layout hints.

Verified: pytest tests/cli_commands/test_create.py -q (121 passed, 6 skipped), ruff check / ruff format --check clean. Manual describe output for bub/default now shows only the {{cookiecutter.project_slug}}/ tree.