feat(cmd): add unified diff output option for formatting

* Implemented `-d`/`--diff` flag to print unified diffs.
* Added `unifiedDiff` function for generating diffs.
* Config discovery now merges fields from `.pgtidy.yaml`.

pkg/config/config.go

@@ -1,9 +1,17 @@
 // Package config defines PgTidy's formatter (and, later, linter) configuration.
 //
-// Defaults encode the project house style reverse-engineered from the corpus.
-// A future change will load/merge these from a discovered .pgtidy.yaml file.
+// Defaults encode the project house style. A .pgtidy.yaml file discovered by
+// walking up from the target directory overrides individual fields.
 package config
 
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"gopkg.in/yaml.v3"
+)
+
 // Case controls keyword/identifier casing.
 type Case string
 
@@ -53,3 +61,101 @@ func Default() Style {
 		Commas:      CommaLeading,
 	}
 }
+
+// yamlFile is the on-disk representation of .pgtidy.yaml.
+// All fields are pointers so we can distinguish "not set" from "set to zero value".
+type yamlFile struct {
+	Indent      *string `yaml:"indent"`
+	Newline     *string `yaml:"newline"`
+	KeywordCase *string `yaml:"keyword_case"`
+	IdentCase   *string `yaml:"ident_case"`
+	TypeCase    *string `yaml:"type_case"`
+	Commas      *string `yaml:"commas"`
+}
+
+// Load discovers and parses the nearest .pgtidy.yaml by walking up from
+// startDir. Fields present in the file override the house-style defaults;
+// missing fields keep the default value. Returns Default() when no config
+// file is found.
+func Load(startDir string) (Style, error) {
+	st := Default()
+	path, err := findConfig(startDir)
+	if err != nil || path == "" {
+		return st, err
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return st, fmt.Errorf("pgtidy: read %s: %w", path, err)
+	}
+
+	var yf yamlFile
+	if err := yaml.Unmarshal(data, &yf); err != nil {
+		return st, fmt.Errorf("pgtidy: parse %s: %w", path, err)
+	}
+
+	if yf.Indent != nil {
+		st.Indent = *yf.Indent
+	}
+	if yf.Newline != nil {
+		st.Newline = *yf.Newline
+	}
+	if yf.KeywordCase != nil {
+		c := Case(*yf.KeywordCase)
+		if err := validCase(c); err != nil {
+			return st, fmt.Errorf("pgtidy: %s: keyword_case: %w", path, err)
+		}
+		st.KeywordCase = c
+	}
+	if yf.IdentCase != nil {
+		c := Case(*yf.IdentCase)
+		if err := validCase(c); err != nil {
+			return st, fmt.Errorf("pgtidy: %s: ident_case: %w", path, err)
+		}
+		st.IdentCase = c
+	}
+	if yf.TypeCase != nil {
+		c := Case(*yf.TypeCase)
+		if err := validCase(c); err != nil {
+			return st, fmt.Errorf("pgtidy: %s: type_case: %w", path, err)
+		}
+		st.TypeCase = c
+	}
+	if yf.Commas != nil {
+		cs := CommaStyle(*yf.Commas)
+		if cs != CommaLeading && cs != CommaTrailing {
+			return st, fmt.Errorf("pgtidy: %s: commas: must be \"leading\" or \"trailing\"", path)
+		}
+		st.Commas = cs
+	}
+
+	return st, nil
+}
+
+// findConfig walks parent directories from startDir looking for .pgtidy.yaml.
+// Returns ("", nil) when no file is found before reaching the filesystem root.
+func findConfig(startDir string) (string, error) {
+	dir, err := filepath.Abs(startDir)
+	if err != nil {
+		return "", err
+	}
+	for {
+		candidate := filepath.Join(dir, ".pgtidy.yaml")
+		if _, err := os.Stat(candidate); err == nil {
+			return candidate, nil
+		}
+		parent := filepath.Dir(dir)
+		if parent == dir {
+			return "", nil
+		}
+		dir = parent
+	}
+}
+
+func validCase(c Case) error {
+	switch c {
+	case CaseUpper, CaseLower, CasePreserve:
+		return nil
+	}
+	return fmt.Errorf("must be \"upper\", \"lower\", or \"preserve\"")
+}