Validation layers

@daggler/validators runs every layer over a parse result and returns typed diagnostics, error, warning and info counts, a security posture score (0 to 100, graded A to F) and a complexity metric.

Layer	Codes	What it checks
0 Parser		Syntax errors and warnings from YAML parsing.
1 Schema	SCHEMA001-005	Required fields, empty step lists, missing runs-on, no jobs, no triggers.
2 Expressions	EXPR001-004	$ context availability: matrix without a strategy, needs.<name> not in needs, steps.<id> that does not exist, inputs without dispatch or call triggers.
3 Graph semantics	SEM001-003, SEM005	needs cycles, dangling references, unreachable jobs, matrix combinations over 50.
4 Actions	ACT001-005	uses: steps checked against the curated action catalog: cache hints, deprecated major versions, missing or unknown inputs, undeclared outputs.

On top of these, the policy engine adds the security rules.