fix(apply): propagate read errors, auto-open repo, dedupe adapter base (#6)

Three robustness fixes for the apply pipeline:

1. Silent read errors on state/target files treated 'file absent' as
   'file empty' and fast-forward-overwrote user edits. Add
   common.ReadExistingFile that returns (nil,nil) only on
   os.IsNotExist; other I/O errors propagate. Use it from apply.Run
   and from every adapter's Render (claudecode, crush, kilo,
   opencode, goose, zed).

2. apply.Run required callers to pre-construct *gitx.Repo; passing
   nil silently skipped the commit. Run now auto-opens a repo from
   Bundle.Root, gracefully no-ops when the root is not a git repo,
   and uses a new gitx.HasStagedChanges() check to skip the no-op
   'nothing to commit' that triggered the original silent-skip.

3. Each of 7 adapter packages re-implemented the same 25-line
   Option/WithHome/New constructor. Hoist into adapter.Base +
   adapter.BaseOption + NewBase. Each adapter embeds *Base and
   reduces to a 9-line constructor. Public New(WithHome(h)) API
   preserved; a.home becomes a.Home.

Tests:
  - apply: 5 new (propagates-read-error, auto-opens-repo,
    no-repo-skips-commit, skips-no-op-commit, commit-message-counts)
  - gitx: 2 new (HasStagedChanges empty/after-add)
  - adapter/common: 3 new (ReadExistingFile missing/content/perm)
  - adapter: 4 new (NewBase default/WithHome/order/typed)
  - existing adapter tests unchanged — verify a.Home still honoured.

internal/gitx/repo_test.go

@@ -41,3 +41,33 @@ func TestShowFileAtHead(t *testing.T) {
 	_, err = r.ShowFileAtHead("nope.txt")
 	assert.Error(t, err)
 }
+
+func TestHasStagedChangesEmptyAfterInit(t *testing.T) {
+	dir := t.TempDir()
+	r := New(dir)
+	require.NoError(t, r.Init())
+	require.NoError(t, r.Configure("nobody", "[email protected]"))
+	// No commits yet, working tree clean.
+	dirty, err := r.HasStagedChanges()
+	require.NoError(t, err)
+	assert.False(t, dirty)
+}
+
+func TestHasStagedChangesAfterAdd(t *testing.T) {
+	dir := t.TempDir()
+	r := New(dir)
+	require.NoError(t, r.Init())
+	require.NoError(t, r.Configure("nobody", "[email protected]"))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "a.txt"), []byte("x"), 0o600))
+	require.NoError(t, r.AddAll())
+
+	dirty, err := r.HasStagedChanges()
+	require.NoError(t, err)
+	assert.True(t, dirty)
+
+	// After commit, index matches HEAD again → no staged changes.
+	require.NoError(t, r.Commit("c"))
+	dirty, err = r.HasStagedChanges()
+	require.NoError(t, err)
+	assert.False(t, dirty)
+}