feat(nvim): migrate gemini-cli plugin to antigravity-cli

[eezzekl]

Closes #180
Related to #173

Summary

• Replaces the deprecated jonroosevelt/gemini-cli.nvim with its official successor mceazy2700/antigravity-cli.nvim (Antigravity CLI, binary agy). Google discontinued Gemini CLI on 2026-05-19 and stopped serving it on 2026-06-18.
• Fixes a latent bug: gemini.lua was never listed in disabled.lua, so despite the docs marking it "Disabled" it always loaded and called require("gemini").setup() at startup, which triggered a blocking vim.fn.input() prompt on clean installs (setup() auto-runs from plugin/gemini.vim, causing double execution and a startup-blocking prompt under lazy.nvim #173). The new spec is lazy-loaded via cmd/keys and is registered in disabled.lua.
• Keeps the repo's mutually-exclusive AI plugin model intact: claudecode.nvim stays the only plugin enabled, antigravity-cli.nvim ships with enabled = false.

Type of change

• Bug fix
• New feature
• Documentation only
• Code refactoring
• Maintenance/tooling
• Breaking change

Changes

File	Change
GentlemanNvim/nvim/lua/plugins/gemini.lua	Removed — plugin is deprecated and eagerly ran setup() at startup
GentlemanNvim/nvim/lua/plugins/antigravity.lua	New spec: lazy-loads on cmd/keys, <leader>a mappings mirroring the Claude Code panel, command = "agy", terminal position = "left", size = 55 (plugin default of 80 columns is too wide)
GentlemanNvim/nvim/lua/plugins/disabled.lua	Registers antigravity-cli.nvim with enabled = false and documents how to switch it on
docs/ai-configuration.md	Gemini rows replaced with Antigravity in the plugin table, CLI install table, and the "which one should I use" table
GentlemanNvim/nvim/lazy-lock.json	Regenerated: drops the gemini-cli.nvim entry, adds antigravity-cli.nvim

Test plan

• Fresh headless sync resolves cleanly: XDG_CONFIG_HOME=<repo>/GentlemanNvim nvim --headless "+Lazy! sync" +qa
• Neovim starts with no blocking prompt (the setup() auto-runs from plugin/gemini.vim, causing double execution and a startup-blocking prompt under lazy.nvim #173 symptom is gone)
• With enabled = false the plugin does not load, matching every other non-default AI plugin in this config
• Exercised with enabled = true in a local Neovim config: the terminal toggle, ask, resume, model-select and diff accept/deny commands all work against agy 1.1.4

Notes

• <leader>a is shared by all AI plugins in this config, so Antigravity is intentionally shipped disabled — only one AI plugin should be enabled at a time, as docs/ai-configuration.md states.
• Opened from a fork, so I cannot apply a type:* label myself. Based on the commit type and the #173 startup regression it fixes, type:bug looks correct — happy to follow whatever you prefer.