Add skill for SQL usage domains (#69)

[knutman]

Closes #69.

What this adds

db/features/usage-domains.md — a practical skill for Oracle SQL usage domains (23ai+), covering:

• All four domain flavors with runnable examples: single-column, enum (both the check-constraint pattern and native ENUM domains), multi-column (currency composite with DOMAIN_DISPLAY/DOMAIN_ORDER over a column list), flexible domains (discriminant column), plus JSON-schema domains and the built-in SYS domains.
• Association semantics: the three attach syntaxes, validation on re-attach, one-domain-per-column (ORA-11512), STRICT vs non-strict (non-strict ignores precision/scale entirely).
• The full lifecycle: ALTER DOMAIN (display/order/annotations only), DROP DOMAIN [FORCE [PRESERVE]] incl. recycle-bin dependents, and a safe recreation sequence for the unalterable parts (data type, check constraints) with the FORCE vs FORCE PRESERVE trade-off.
• A dictionary/function reference table (DOMAIN_CHECK, DOMAIN_CHECK_TYPE, CAST ... AS DOMAIN, the USER_DOMAIN_* views) and ten best practices, including the PL/SQL limitation (no domain-typed variables or parameters — type safety ends at the table).
• ## Oracle Version Notes (19c baseline: feature absent; 23ai vs newer release updates) and a ## Sources section.

Also updates the db/features/ row in db/SKILL.md (category routing).

How it was validated

• Syntax and semantics checked against the CREATE/ALTER/DROP DOMAIN pages of the Oracle Database 26 SQL Language Reference and the domain functions reference (linked in ## Sources).
• Cross-checked against Oracle-Base's 23ai domains article and Chris Saxon's UKOUG 2023 slides.
• Statements exercised hands-on against Oracle Database 23ai (version 23.26); observations that go beyond the documentation (silent no-op when detaching a single column, recycle-bin rows blocking DROP DOMAIN, the NAME column in USER_DOMAIN_CONSTRAINTS) are explicitly marked as verified on that version.

Review steps

1. Open db/features/usage-domains.md, pick any example and run it against a 23ai instance (23.7+ for ALTER DOMAIN/native ENUM).
2. Check the routing line in db/SKILL.md renders correctly.