fix(config): validate matrix dimension keys so they cannot red the pipeline at GHA parse

[joshua-temple]

Matrix dimensions keys were the only emitted identifier class with no validation, so a manifest with a key like 'go version' (a space) passed cascade validation but redded the generated pipeline at first GitHub Actions parse (the key lands raw as a YAML key and in ${{ matrix. }}).

• validateMatrix (internal/config/validate_v1.go) rejects any dimension key that is not a valid GHA matrix identifier (^[A-Za-z_][A-Za-z0-9_-]*$ - hyphens kept, since ${{ matrix.node-version }} is the canonical GHA pattern) and rejects empty axes, naming the offending key.
• The same constraint (propertyNames pattern + minItems) is added to all three byte-identical schema copies.

Verification: build/-race/lint green, 3015 tests; 13-case red-then-green validation test; e2e scenario 63-build-matrix exercises a valid matrix (incl. a hyphenated axis) and actionlints clean; no shipped manifest sets dimensions.

[joshua-temple]

Superseded by the rebuilt PR off current main (scenario renumbered to avoid collision).