Add erdn sql command for SQL DDL export with multi-DBMS support

[Copilot]

Adds a new sql subcommand that generates SQL DDL from an .erdn schema, targeting MySQL, PostgreSQL, MSSQL, Oracle, or SQLite via --dbms.

New command

erdn sql <schema.erdn> [--dbms <mysql|postgresql|mssql|oracle|sqlite>] [--out <file.sql>]

Defaults to mysql. Output file defaults to <schema>.sql.

What gets generated

• CREATE TABLE with DBMS-appropriate column types
• PRIMARY KEY table constraints
• Auto-increment per dialect: AUTO_INCREMENT (MySQL), IDENTITY(1,1) (MSSQL), GENERATED ALWAYS AS IDENTITY (PostgreSQL/Oracle), INTEGER PRIMARY KEY AUTOINCREMENT (SQLite)
• CREATE INDEX for indexed columns
• Foreign keys from link declarations — ALTER TABLE … ADD CONSTRAINT … FOREIGN KEY for most DBMSes; inline FOREIGN KEY table constraints for SQLite (with PRAGMA foreign_keys = ON)

Type mapping highlights

erdn type	MySQL	PostgreSQL	MSSQL	Oracle	SQLite
bool	TINYINT(1)	BOOLEAN	BIT	NUMBER(1)	INTEGER
varchar(n)	VARCHAR(n)	VARCHAR(n)	NVARCHAR(n)	VARCHAR2(n)	TEXT
text	TEXT	TEXT	NVARCHAR(MAX)	CLOB	TEXT
bigint	BIGINT	BIGINT	BIGINT	NUMBER(19)	INTEGER
uuid	CHAR(36)	UUID	UNIQUEIDENTIFIER	CHAR(36)	TEXT

NOW() defaults are translated per dialect (GETDATE() for MSSQL, SYSDATE for Oracle, CURRENT_TIMESTAMP for SQLite). Unknown types pass through uppercased.

Changes

• internal/sqlexport/sqlexport.go — new package; all generation logic isolated here
• internal/sqlexport/sqlexport_test.go — tests covering all five DBMSes, type mappings, self-referential links, default value translation, indexes, and FK output
• cmd/erdn/main.go — wires runSQL() into the subcommand dispatcher
• README.md / website/guide.md — documents the new command and DBMS table

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