ci: matrix over Node 22 and 24, add the bench and packaging tiers (task 0040)

The workflow pinned Node 20, which reached end-of-life in April 2026, while
the manifest promised support down to it — so the entire claimed range below
the single tested version went unverified and its floor was unsupported.

Matrix over the two supported long-term-support majors and narrow the engine
range to `^22 || ^24`, naming exactly what is tested. Narrowing is free now
because nothing has been published and no tags exist.

The benchmark harness tier joins every leg: it is deterministic and needs no
network, and its non-default runner configuration made it easy to believe it
was running when it was not. The end-to-end and packaging tiers run on the
highest leg only, conditioned on a flag attached to that leg through a matrix
`include` entry rather than on a version number restated at each site. The
benchmark smoke tier stays out, since it would pass by skipping.

`@types/node` follows the new floor; it was the last Node 20 reference in the
manifest, and the typecheck runs on every leg.

.claude/spec/gitea-axi.md

@@ -39,7 +39,7 @@ It ships as both an installable npm CLI and a bundled Agent Skill, so any agent
 
 ### Language and Runtime
 
-TypeScript on Node 20+, matching the `gh-axi` reference implementation.
+TypeScript on the supported Node long-term-support majors — currently 22 and 24, as declared in the manifest's engine range and matrixed over by continuous integration.
 ESM module format.
 
 ### Implementation Strategy