feat(postgres): initial stable release v0.9.0

pgx v5-native PostgreSQL client with launcher lifecycle, health check, unit-of-work via context injection, and structured error mapping.

What's included:
- Executor / Tx / Client / Component interfaces using pgx native types (pgconn.CommandTag, pgx.Rows, pgx.Row)
- New(logger, cfg) constructor; pgxpool initialised in OnInit
- Config struct with env-tag support for all pool tuning parameters
- UnitOfWork via context injection; GetExecutor(ctx) returns active Tx or pool
- HandleError mapping pgerrcode constants to xerrors codes (AlreadyExists, InvalidInput, NotFound, Internal)
- health.Checkable at LevelCritical; HealthCheck delegates to pgxpool.Ping

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

README.md

@@ -0,0 +1,66 @@
+# postgres
+
+pgx-backed PostgreSQL client with launcher lifecycle and health check integration.
+
+## Install
+
+```
+go get code.nochebuena.dev/go/postgres
+```
+
+## Usage
+
+```go
+db := postgres.New(logger, cfg)
+lc.Append(db)
+r.Get("/health", health.NewHandler(logger, db))
+```
+
+## Querying
+
+```go
+exec := db.GetExecutor(ctx) // returns pool, or active Tx if inside Do()
+
+rows, err := exec.Query(ctx, "SELECT id, name FROM users WHERE active = $1", true)
+defer rows.Close()
+```
+
+## Unit of Work
+
+```go
+uow := postgres.NewUnitOfWork(logger, db)
+
+err := uow.Do(ctx, func(ctx context.Context) error {
+    exec := db.GetExecutor(ctx) // returns the transaction
+    _, err := exec.Exec(ctx, "INSERT INTO orders ...")
+    return err
+})
+```
+
+## Error mapping
+
+```go
+if err := db.HandleError(err); err != nil { ... }
+// or package-level: postgres.HandleError(err)
+```
+
+| PostgreSQL error | xerrors code |
+|---|---|
+| `unique_violation` | `ErrAlreadyExists` |
+| `foreign_key_violation` | `ErrInvalidInput` |
+| `check_violation` | `ErrInvalidInput` |
+| `pgx.ErrNoRows` | `ErrNotFound` |
+| anything else | `ErrInternal` |
+
+## Configuration
+
+| Env var | Default | Description |
+|---|---|---|
+| `PG_HOST` | required | Database host |
+| `PG_PORT` | `5432` | Database port |
+| `PG_USER` | required | Username |
+| `PG_PASSWORD` | required | Password |
+| `PG_NAME` | required | Database name |
+| `PG_SSL_MODE` | `disable` | SSL mode |
+| `PG_MAX_CONNS` | `5` | Max pool connections |
+| `PG_MIN_CONNS` | `2` | Min pool connections |