docs: describe tag and branch retention as Lance cleanup implements it

The cleanup and guarantees pages said a tag or branch pointing at a pruned
version stops resolving, while the table page said tagged versions are
exempt and quietly kept. The pinned Lance (ae2903729f02, the commit in
Cargo.lock and in Sophon's Lance submodule) keeps a tagged version
whatever its age, but with its default policy refuses to clean up while a
tag points at a version older than the window, raising an error that
names the tag. optimize() passes that default, so it raises after
compacting, prunes nothing and skips its index update until the tag is
deleted. Sophon's cleanup job uses the same default.

Branches are retained separately: cleaning the parent keeps the version a
branch was forked from and the files the branch reads, while the
parent's other old versions are pruned. The pages now say both, checked
locally against the baseline SDK.

docs/web/basics/guarantees.mdx

@@ -74,9 +74,11 @@ exists.
 <Warning>
 Retention is finite. [Cleanup](/maintenance/cleanup) prunes versions older
 than the retention window — **7 days by default** for `optimize()` in the
-embedded library, while a deployment's configuration sets its own — and a tag or branch
-pointing at a pruned version no longer resolves. If you rely on reading old
-versions, widen the window before running it, not after.
+embedded library, while a deployment's configuration sets its own. A tag keeps
+its version, and while a tag points at a version older than the window, cleanup
+refuses to run until the tag is deleted; a branch keeps the version it was
+forked from. If you rely on reading old versions, widen the window before
+running it, not after.
 </Warning>
 
 **Dropping a table ends its history.** Versioning does not survive