feat(import): reverse-sync legacy rules via cinch import

[00200200]

Summary

• Adds cinch import <path> [--out DIR] [--from-harness/--from] [--dry-run] [--yes] to decompile legacy Cursor, Claude, Copilot, and Cline rule/skill files into canonical Cinch SKILL.md packages under .cinch/imported by default.
• Reuses the existing Doc model and frontmatter helpers; auto-detects dialect from path markers and requires --from-harness when ambiguous.
• Covers unit tests for each dialect plus Cline/Claude round-trip import(export(skill)).

Closes #7

Test plan

• uv run pytest
• uv run ruff check .
• uv run ruff format --check .
• Manually: cinch import .cursorrules --dry-run then --yes and inspect .cinch/imported

[github-actions]

👋 Thanks for opening this pull request, @00200200!

We're thrilled to have your contribution to cinch! Here is a quick checklist to help get your PR merged smoothly:

• Linked Issue: Issue reference detected in PR body. ✅
• Local Verification: Verify all checks pass locally using uv run pytest and uv run ruff check ..
• Commit Hygiene: Ensure commit messages follow conventional commit style (feat:, fix:, docs:).

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