Revert surround keymaps to nvim-surround defaults (ds/cs)

## Goal
Remove the Graphite translation layer for surround delete/change operations.
Use nvim-surround defaults (`ds`, `cs`) instead of custom `xs`, `ws` mappings.

## Why
The `ws` mapping has known layering issues (`w→c` remap intercepts before `ws` is recognized).
Using defaults is simpler, more consistent with documentation/tutorials, and avoids the complexity of maintaining a translation layer for surround operations.

## Changes Required

### 1. `lua/plugins/surround.lua`
- Remove Graphite `ws` → `(nvim-surround-change)` mapping (line 135)
- Remove Graphite `xs` → `(nvim-surround-delete)` mapping (line 136)
- Remove `xst` feedkeys wrapper (lines 137-139)
- Remove `xs`, `ws` from buffer exclusion `keymaps_to_disable` list (line 157)
- Remove the TODO comment block about `ws` layering issues (lines 129-134)

### 2. `lua/config/keymaps-surround.lua`
- Remove `x.s` group (delete surround via Graphite, lines 35-48)
- Remove `w.s` group (change surround via Graphite, lines 49-62)
- Keep `d.s` and `c.s` groups (these document the defaults)
- Keep `y.s` group (add surround — already default)

### 3. `lua/config/tests/surround/test.lua`
- Remove `describe("graphite mappings (xs, ws)")` test block (lines 282-299)
- Keep all `ds`, `cs`, `ys` default tests (they already pass)

### 4. `.claude/skills/nvim-config/references/surround.md`
- Remove "Delete (Graphite: X=delete)" section with `xs`/`xst`
- Remove "Change (Graphite: W=change)" section with `ws`
- Remove "Known Issues" section about `ws` layering
- Keep default `ds`, `cs`, `ys` documentation

### 5. `lua/config/keymaps-old.lua`
- No changes needed (the `w→c` and `x→d` global remaps stay — they just won't be used for surround anymore)

## Result
- `ds{char}` = Delete surround (nvim-surround default)
- `cs{old}{new}` = Change surround (nvim-surround default)
- `ys{motion}{char}` = Add surround (already default)
- `S{char}` = Visual surround (already default)

## Supersedes
Closes #54