feat(highlight): wire highlights.context into treesitter pipeline (#151)

## Problem

`highlights.context.enabled` and `highlights.context.lines` were
defined, validated, and range-checked but never read during
highlighting. Hunks inside incomplete constructs (e.g., a table literal
or function body whose opening is beyond the hunk's own context lines)
parsed incorrectly because treesitter had no surrounding code.

## Solution

`compute_hunk_context` in `init.lua` reads the working tree file using
the hunk's `@@ +start,count @@` line numbers to collect up to `lines`
(default 25) surrounding code lines in each direction. Files are read
once via `io.open` and cached across hunks in the same file.
`highlight_treesitter` in `highlight.lua` accepts an optional context
parameter that prepends/appends context lines to the parse string and
offsets capture rows by the prefix count, so extmarks only land on
actual hunk lines. Wired through `highlight_hunk` for the two
code-language treesitter calls (not headers, not `highlight_text`, not
vim syntax).

Closes #148.

lua/diffs/parser.lua

@@ -15,6 +15,8 @@
 ---@field prefix_width integer
 ---@field quote_width integer
 ---@field repo_root string?
+---@field context_before string[]?
+---@field context_after string[]?
 
 local M = {}