Add custom table for querying custom directives

[yeatse]

Summary

Adds a custom table exposing Beancount custom directives, with the same shape as the existing notes / events / documents tables.

Columns:

Column	Type	Description
meta	dict	Directive metadata (filename, lineno, …)
date	date	Directive date
type	str	The custom directive type string
values	list	Typed values as parsed by Beancount

Example::

SELECT date, type, values FROM custom WHERE type = 'budget' ORDER BY date

Test plan

• New beanquery/custom_test.py covers: column schema, SELECT * wildcard excludes meta, row contents with mixed value types (string / Amount / date), WHERE type = filtering, meta contents, empty ledger
• Updated shell_test.py .tables expectation
• Full suite: 299 passed

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