refactor(shared-symlinks): selective .claude subdirectory symlinking and eliminate duplication

[fx]

Summary

Refactors the shared symlinks setup script to intelligently handle .claude directory symlinking and eliminates code duplication. Adds comprehensive Docker-based test suite with CI integration.

Changes

Selective .claude Symlinking

• Fresh install (no ~/.claude): Symlinks entire directory → all content accessible
• Existing ~/.claude: Only symlinks specific subdirectories:
  • agents/
  • commands/
  • skills/
  • CLAUDE.md
• Preserves user's existing files (settings, custom content, etc.)

Code Refactoring

• Extracted .claude symlinking logic into symlink_claude() function
• Extracted general symlinking logic into symlink_to_home() function
• Eliminated ~40 lines of duplicated code
• Clarified that /shared/ is Coder-only with better comments

Test Suite

• Added test/test-dotfiles.sh with 3 Docker-based tests:
  1. Non-Coder environment (direct symlink)
  2. Coder fresh install (full directory symlink to /shared/)
  3. Coder existing ~/.claude (selective subdirectory symlinks)
• Added GitHub Actions CI workflow (.github/workflows/test.yml)
• Updated CLAUDE.md with testing documentation

Bug Fixes

• Added skills/ directory to shared/claude/
• Fixed ln command to use -n flag (prevents creating symlinks inside existing directories)

Test Results

All tests pass locally:

✓ Non-Coder test passed
✓ Coder fresh install test passed
✓ Coder existing ~/.claude test passed
✓ All tests passed!

Testing Instructions

# Start Docker if needed
sudo service docker start

# Run tests locally
./test/test-dotfiles.sh

[Copilot]

Pull Request Overview

This PR adds comprehensive automated testing infrastructure for the dotfiles installation system. The tests verify correct behavior in both Coder workspace and non-Coder environments, ensuring symlink handling works properly for the .claude directory and related configurations.

• Introduces a Docker-based test script (test/test-dotfiles.sh) that validates three installation scenarios
• Adds GitHub Actions CI workflow for automated testing on push and pull requests
• Refactors .chezmoiscripts/run_onchange_after_setup-shared-symlinks.sh.tmpl to extract reusable functions for symlink management
• Documents the testing approach and procedures in CLAUDE.md

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/test-dotfiles.sh	New test script that validates dotfiles installation in non-Coder, Coder fresh install, and Coder existing directory scenarios using Docker
.github/workflows/test.yml	CI workflow configuration to run tests automatically on main branch and pull requests
.chezmoiscripts/run_onchange_after_setup-shared-symlinks.sh.tmpl	Refactored to extract symlink_claude() and symlink_to_home() functions, improving maintainability and reusability
CLAUDE.md	Added comprehensive testing documentation covering local and CI testing procedures
shared/claude/skills/README.md	Added placeholder README for the skills directory

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