fix: don't exit with code 1 when there is nothing to review

[scastlara]

It was surprising behavior, and some TT people complained to me (understandably).

This PR contains some additional cosmetic changes (docstrings and internal renames that don't matter).

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
lgtm_ai
__main__.py	152	15	90%	55, 168, 221–223, 241–242, 278–280, 303, 327–328, 330–331
lgtm_ai/config
handler.py	143	1	99%	219
lgtm_ai/formatters
base.py	10	0	100%
json.py	16	2	87%	22, 26
markdown.py	51	0	100%
pretty.py	38	10	73%	18, 48, 62, 75–76, 83, 87, 95, 99, 105
lgtm_ai/git
parser.py	53	2	96%	90–91
lgtm_ai/git_client
gitlab.py	159	20	87%	42–44, 63–64, 88–89, 95–96, 112–114, 123–124, 129–130, 133, 212, 245–246
lgtm_ai/jira
jira.py	65	3	95%	37–39
lgtm_ai/mcp
__main__.py	39	7	82%	4–5, 54, 87, 90–91, 95
lgtm_ai/review
context.py	106	18	83%	66–68, 72, 97, 129–130, 133–134, 145, 148–149, 157–162
guide.py	35	1	97%	38
reviewer.py	61	4	93%	96–98, 100
TOTAL	1799	146	91%

Tests	Skipped	Failures	Errors	Time
237	1 💤	0 ❌	0 🔥	10.755s ⏱️