feat(conflict): add virtual text formatting and action lines (#101)

## Problem

Conflict resolution virtual text only showed plain "current" /
"incoming"
labels with no keymap hints. Users had no way to discover available
resolution keymaps without reading docs.

## Solution

Default virtual text labels now include keymap hints: `(current — doo)`
and
`(incoming — dot)`. A new `format_virtual_text` config option lets users
customize or hide labels entirely. A new `show_actions` option (off by
default) renders a codelens-style action line above each `<<<<<<<`
marker
listing all enabled resolution keymaps. Merge diff views also gain hunk
hints on `@@` header lines showing available keymaps.

New config fields: `conflict.format_virtual_text` (function|nil),
`conflict.show_actions` (boolean). New highlight group:
`DiffsConflictActions`.

README.md

@@ -9,18 +9,12 @@ syntax highlighting.
 
 ## Features
 
-- Treesitter syntax highlighting in `:Git` diffs and commit views
-- Diff header highlighting (`diff --git`, `index`, `---`, `+++`)
-- `:Gdiffsplit` / `:Gvdiffsplit` syntax through diff backgrounds
-- `:Gdiff` unified diff against any git revision with syntax highlighting
-- Fugitive status buffer keymaps (`du`/`dU`) for unified diffs
-- Background-only diff colors for any `&diff` buffer (`:diffthis`, `vimdiff`)
-- Vim syntax fallback for languages without a treesitter parser
-- Hunk header context highlighting (`@@ ... @@ function foo()`)
-- Character-level (intra-line) diff highlighting for changed characters
-- Inline merge conflict detection, highlighting, and resolution keymaps
-- Configurable debouncing, max lines, diff prefix concealment, blend alpha, and
-  highlight overrides
+- Treesitter syntax highlighting in fugitive diffs and commit views
+- Character-level intra-line diff highlighting
+- `:Gdiff` unified diff against any revision
+- Background-only diff colors for `&diff` buffers
+- Inline merge conflict detection, highlighting, and resolution
+- Vim syntax fallback, context padding, configurable blend/debounce
 
 ## Requirements