refactor: replace plenary with built-in Neovim APIs

[kyoh86]

Summary

• replace the current plenary usage with built-in Neovim APIs and direct stdlib usage
• remove plenary.nvim from runtime, test, health, and flake dependencies
• update docs to reflect Neovim >= 0.10.0

Notes

• this does not add an internal compatibility layer for plenary
• the goal is to use vim.system, vim.fs, vim.uv, jobstart, and direct Lua/Neovim APIs where they are already sufficient

Verification

• make format
• make test
• nvim --headless --noplugin -u NONE -i NONE -c "set rtp+=." -c "lua require('mcphub')" -c qa
• make docs was not run here because pandoc is not installed in this environment; doc/mcphub.txt was updated manually to keep docs in sync

Closes #284

[kyoh86]

Keeping this as a draft for now because the discussion in #284 is not settled yet.\n\nThe implementation is here so the approach is concrete and reviewable, but I do not want to treat the direction as agreed before the issue discussion concludes.