feat(terminal): support top and bottom split_side

[giuxtaposition]

Summary

Adds top and bottom to terminal.split_side (previously left/right only). Horizontal splits reuse terminal.split_width_percentage (and diff-time terminal.diff_split_width_percentage) as a fraction of vim.o.lines instead of vim.o.columns. Diff opening, closing, and new-tab teardown all pick the correct axis and wincmd per side.

• Zero-config for existing users: left/right behavior unchanged.
• Native provider: single create_split helper picks split/vsplit and topleft/botright per side; replaces two near-duplicate placement blocks in open_terminal and show_hidden_terminal.
• Diff resize: resize_terminal_for_diff reads split_side and calls nvim_win_set_height for top/bottom, nvim_win_set_width for left/right.
• New-tab diffs: wincmd move (H/L/K/J) and post-layout jump (h/l/j/k) picked from a small lookup table.
• Small readability cleanup: rename resolve_split_width_percentage → resolve_terminal_split_fraction (config keys keep their historical names; public API unchanged).
• types.lua: new ClaudeCodeSplitSide alias covers all four values.

Usage

require("claudecode").setup({
  terminal = {
    split_side = "bottom",           -- "left" | "right" | "top" | "bottom"
    split_width_percentage = 0.30,   -- applied to lines for top/bottom, columns for left/right
    diff_split_width_percentage = 0.20, -- optional shrink while a diff is open
  },
})

Test plan

• mise run check and mise run test.
• Set split_side = "bottom" → Claude terminal opens as a bottom horizontal split sized to lines * split_width_percentage.
• Set split_side = "top" → opens as a top horizontal split; topleft placement confirmed.
• Trigger a Claude edit with split_side = "bottom" → diff opens, terminal shrinks to diff_split_width_percentage in height.
• Accept/reject the diff → terminal restores to idle height, focus returns to expected window.
• open_in_new_tab = true with split_side = "bottom" → new tab lays out with terminal below, diff above; jump lands in the diff pane.
• Regression: split_side = "left" / "right" behavior unchanged (width-based).

Related issues

Closes #153

Some screenshots