Assess reuse of `reason` for the fields listed in issue #385

[colmjude]

Follow-on from issue #385, specifically the reason field rationalisation candidates comment.

Assess whether these fields should reuse reason, or remain separate:

• use-lawful-reason
• removal-reasons
• bng-condition-exemption-reasons

For each field, check:

• whether it means the same thing as reason in practice
• whether module context changes the meaning
• whether the data shape is compatible, especially for bng-condition-exemption-reasons

Outcome:

• a recommendation for each field
• any follow-on changes needed if reuse is recommended

[colmjude]

field	current module	rationalise to reason?	plain English rationale
use-lawful-reason	grounds-existing-use	Yes	This is still a reason field in practice, but the module needs to keep the lawful-development wording so the legal context stays clear.
removal-reasons	hedgerow-removal	Yes	This is a straightforward reason field. The module already gives the hedgerow context, so reuse looks sensible.
bng-condition-exemption-reasons	bng	No	This is not a single reason field. It is a list of structured exemption entries, and each entry already includes its own reason.

[colmjude]

Done, see commit 630334