GeezerVison

What

Document the SQLCDEBUG=databases=managed setting on the environment-variables reference page.

Why / evidence

internal/sqlcdebug/sqlcdebug.go registers databases in its master Settings
table ("set to 'managed' to disable database connections via URI"), and
internal/cmd/vet.go reads it (debugDatabases = sqlcdebug.New("databases"),
OnlyManagedDB: debugDatabases.Value() == "managed") to reject uri databases
during sqlc vet. The option shipped in #2898 and appears in the changelog, but
docs/reference/environment-variables.md documents every other SQLCDEBUG key
(dumpast, dumpcatalog, trace, processplugins, dumpvetenv,
dumpexplain) except this one.

Change

Adds a ### databases subsection under ## SQLCDEBUG, matching the style of the
surrounding entries. Docs-only, one file.

Verified

Built sqlc at HEAD (go build -o /tmp/sqlc-dev ./cmd/sqlc, reports v1.31.1)
and ran sqlc vet on a config with a database.uri and the sqlc/db-prepare
rule:

• without the env var: failed to connect to user=postgres database=nope ...
  (sqlc attempts the URI connection)
• SQLCDEBUG=databases=managed sqlc vet:
  database: connections disabled via SQLCDEBUG=databases=managed

go test ./internal/sqlcdebug/ passes.