nvim: migrate treesitter setup

[mandaiy]

What changed

• switch nvim-treesitter and nvim-treesitter-textobjects to the main branch API
• replace the old nvim-treesitter.configs setup with the new require("nvim-treesitter") setup and parser installation flow
• document the tree-sitter-cli requirement in README.md and add it to the Docker image build

Why

The Treesitter config was still using the previous setup style. Moving to the current main branch requires the new API and an explicit tree-sitter-cli dependency so fresh environments can install parsers successfully.

Impact

Neovim will use the new Treesitter setup path, and documented/bootstrap environments will include the CLI needed for parser installation.

Validation

• started Neovim headlessly with XDG_CONFIG_HOME=/Users/mandai/.dotfiles XDG_STATE_HOME=/tmp XDG_CACHE_HOME=/tmp XDG_DATA_HOME=$HOME/.local/share nvim --headless '+q'
• local pre-commit hooks ran during git commit and passed, including stylua

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve tree-sitter indentation setup during migration

This migration drops tree-sitter indentation that was previously enabled via indent = { enable = true }. In the new main-branch API, vim.treesitter.start(...) only starts parsing/highlighting; indentation must be enabled separately (e.g., by setting vim.bo.indentexpr to require('nvim-treesitter').indentexpr()). With only the current callback, users lose tree-sitter-based indentation behavior for filetypes that previously had it.

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