[everbird]

Make the compatibility checks from #1 work with macOS editor launchers, aliased temporary paths, and the system Vim's embedded Python.

Fixes

• Compare installation targets using file identity. On macOS, /var and /private/var can refer to the same directory, so comparing a resolved path against an unresolved path incorrectly failed successful installs.
• Restrict tool discovery through --cmd inside the editor. Homebrew's Vim launcher needs basename, dirname, and readlink before Vim starts; clearing its process PATH prevented the missing-tool tests from reaching Vim.
• Use the runner's existing Python for the standard-library-only tests. setup-python changed LD_LIBRARY_PATH, causing Ubuntu's Vim to mix the hosted Python library with the system standard library. UltiSnips then failed with ModuleNotFoundError: No module named 'math'.
• Include UltiSnips initialization diagnostics in test failures, and run push checks on master while feature branches are checked through their PRs to avoid duplicate matrices.

Validation

• Reproduced all four macOS test failures on Linux with a symlinked temporary directory and a Vim launcher that requires shell utilities; the full 27-test suite passes after the fixes.
• Also passed all applicable tests on Vim 7.4 and Neovim 0.9.5, 0.11.5, and 0.12.6 using the aliased temporary directory. The LSP handshake remains skipped on editors that do not support the configured client.
• Passed actionlint and git diff --check.
• All seven jobs in the GitHub Linux/macOS compatibility matrix passed, including Ubuntu Vim with UltiSnips and both macOS editors.