fix: remove nvim-treesitter healthcheck dependency

[noir4y]

Summary

• remove nvim-treesitter from the documented dependency list and installation examples
• replace the healthcheck dependency probe with parser-availability checks based on Neovim's built-in Tree-sitter APIs
• make make health print the health report in headless runs

Why

nvim-treesitter should no longer be presented as a dependency for this plugin. The healthcheck now focuses on whether parser support is actually available for the target buffer instead of whether a specific plugin is installed.

Closes #8