docs(data-modeling): document declarative triggers via ORM SDK

[pyramation]

Summary

Documents the declarative trigger catalog (shipped in constructive-io/constructive-db#2414) as ORM SDK–only API docs, per this repo's "document the SDK surface, never raw SQL" rule. Custom triggers are now authorable via db.triggerFunction.create + db.trigger.create — this teaches agents that surface.

Authoritative home is constructive-data-modeling (alongside constraints/indexes):

• New references/triggers.md — full reference: two-step flow (function first, then trigger), field table (timing, events[], forEach, transitionOldName/transitionNewName, whenClause), transition tables + statement-level, and the whenClause condition DSL (shared with JobTrigger/EventTracker).
• SKILL.md — new "Triggers" section + whenClause/transition trigger phrases in the description, a "When to Apply" bullet, and a references-table row.

Example documented:

db.triggerFunction.create({ data: { databaseId, name: 'audit_fn', code: 'BEGIN RETURN NULL; END' } })
db.trigger.create({ data: {
  databaseId, tableId, name: 'audit', functionName: 'audit_fn',
  timing: 'after', events: ['update'], forEach: 'statement',
  transitionOldName: 'o', transitionNewName: 'n',
  whenClause: { field: 'status', op: 'IS DISTINCT FROM', row: 'OLD' },
} })
// → CREATE TRIGGER audit AFTER UPDATE ON t
//     REFERENCING OLD TABLE AS o NEW TABLE AS n
//     FOR EACH STATEMENT EXECUTE FUNCTION audit_fn();

Also: a router row in constructive-features/SKILL.md, a "Custom Triggers" subsection in root features.md, and repackaged constructive-data-modeling.zip / constructive-features.zip.

Note: a trigger row without functionName is registration-only and is documented as not provisioned, matching the backend behavior that keeps existing catalog rows/generators unaffected.

Link to Devin session: https://app.devin.ai/sessions/3876baca530b465f9aec29a94eeb413b
Requested by: @pyramation

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