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`.

docs/config/default.pgtidy.yaml

@@ -0,0 +1,26 @@
+# PgTidy house style — all fields shown with their default values.
+# Place as .pgtidy.yaml in your project root (or any parent directory).
+# Any field you omit keeps its default.
+
+# Indentation string for one level (two spaces).
+indent: "  "
+
+# Line terminator written by the formatter.
+newline: "\n"
+
+# Casing for SQL keywords (SELECT, FROM, WHERE, …).
+# upper | lower | preserve
+keyword_case: upper
+
+# Casing for unquoted identifiers (column names, variable names, …).
+# upper | lower | preserve
+ident_case: lower
+
+# Casing for built-in type names (text, integer, boolean, …).
+# upper | lower | preserve
+type_case: lower
+
+# Comma placement in multi-line parameter / column lists.
+# leading  → comma at the start of the continuation line  (,col)
+# trailing → comma at the end of the preceding line       (col,)
+commas: leading

docs/config/lowercase.pgtidy.yaml

@@ -0,0 +1,7 @@
+# Fully lowercase style — keywords, types, and identifiers all lowercased.
+# Common in teams that prefer minimal visual noise.
+
+keyword_case: lower
+ident_case: lower
+type_case: lower
+commas: leading

docs/config/preserve-case.pgtidy.yaml

@@ -0,0 +1,8 @@
+# Preserve-case style — no casing changes applied.
+# Useful for codebases with mixed-convention legacy SQL that you want to
+# reformat structurally (indentation, commas) without touching casing.
+
+keyword_case: preserve
+ident_case: preserve
+type_case: preserve
+commas: leading

docs/config/trailing-commas.pgtidy.yaml

@@ -0,0 +1,7 @@
+# Trailing-comma style — comma at the end of each line rather than the start.
+# Matches the SQL style common in tools like dbt and some BI platforms.
+
+keyword_case: upper
+ident_case: lower
+type_case: lower
+commas: trailing

docs/todo.md

@@ -69,8 +69,10 @@ Legend: ✅ done · 🚧 in progress · ⬜ not started
 - **Status:** all tests pass; idempotence verified.
 
 ### ✅ Printer + style config — `pkg/format`, `pkg/config`
-- `pkg/config`: `Style` struct + `Default()` = house style (UPPERCASE keywords, lowercase
-  types, 2-space indent, leading commas, spacing rules).
+- `pkg/config`: `Style` struct + `Default()` = house style. `Load(startDir)` walks up the
+  directory tree to find `.pgtidy.yaml` and merges its fields over the defaults.
+  Supported keys: `indent`, `newline`, `keyword_case`, `ident_case`, `type_case`, `commas`.
+  Dependency: `gopkg.in/yaml.v3`.
 - `pkg/format`: formats CREATE FUNCTION/PROCEDURE **headers** to house style (params
   one-per-line leading-comma, option clauses each on own line, AS/`$$` own lines); DECLARE
   section formatted (see body parser entry); `Raw` statements emitted verbatim.
@@ -85,7 +87,9 @@ Legend: ✅ done · 🚧 in progress · ⬜ not started
 
 ### ✅ CLI `fmt` + safety harness — `cmd/pgtidy`
 - `pgtidy fmt` (gofmt model): default stdin→stdout; `-w`/`--write`, `-l`/`--list`,
-  `--check` (CI exit codes); `version`/`help`. _`.pgtidy.yaml` discovery + `-d` diff: TODO._
+  `--check` (CI exit codes); `-d`/`--diff` (unified diff output); `version`/`help`.
+- Config discovery: walks up from cwd to find `.pgtidy.yaml`; applied before formatting.
+- `diff.go`: in-house unified diff (LCS-based, zero additional deps).
 - `fmt_test.go`: stdin, --check (un/formatted), -w idempotence, unknown-command.
 - Safety invariants #2 (semantic equivalence), #3 (idempotence), #4 (graceful degradation)
   are tested in `pkg/format` over the corpus.