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/adapter/common/io.go

@@ -29,6 +29,19 @@ func ReadIfExists(path string) (string, error) {
 	return ReadIfExistsFS(fsx.OS(), path)
 }
 
+// ReadExistingFile returns the file body or (nil, nil) when missing.
+// Other errors (permission denied, I/O failure) are returned verbatim so
+// read-modify-write callers can distinguish a fresh first-run state from a
+// broken read. Treating every error as "file is empty" silently overwrites
+// the user's on-disk edits during apply.
+func ReadExistingFile(path string) ([]byte, error) {
+	b, err := os.ReadFile(path) //nolint:gosec // caller-controlled path
+	if err != nil && os.IsNotExist(err) {
+		return nil, nil
+	}
+	return b, err
+}
+
 // DirExistsFS reports whether path is an existing directory on fs.
 func DirExistsFS(fs fsx.FS, path string) bool {
 	info, err := fs.Stat(path)