feat(validator): accept --phase readiness as standalone CLI option

[xdu31]

Summary

• Re-enable --phase readiness as a selectable CLI option so users can validate recipe constraints against the snapshot before deploying
• Single shared runReadinessPhase() replaces the old checkReadiness(), serving both explicit and implicit paths
• --phase all behavior unchanged: readiness runs as an implicit pre-flight gate

Motivation

V2 removed --phase readiness as a selectable option, breaking the CUJ1 workflow where users validate readiness standalone before deploying. This PR adds it back without duplicating logic.

Behavior

CLI	Readiness behavior
--phase readiness	Runs readiness only, returns CTRF report, no cluster access
--phase readiness,deployment	Runs readiness first; if it fails, skips container phases
--phase all (default)	Readiness runs implicitly as a gate; failure returns error before any Jobs deploy

Changes

File	Change
pkg/validator/phases.go	Add PhaseReadiness constant
pkg/validator/validator.go	Replace checkReadiness() with runReadinessPhase(), update ValidatePhases()
pkg/cli/validate.go	Accept "readiness" in CLI parser, update usage strings
pkg/cli/validate_test.go	Change "readiness is invalid" test to expect success
pkg/validator/validator_test.go	Replace checkReadiness tests with runReadinessPhase equivalents

Test plan

• go test -race ./pkg/validator/... passes
• go test -race ./pkg/cli/... passes
• --phase readiness accepted by CLI parser
• --phase all behavior unchanged (implicit readiness gate)
• Nil/empty constraint edge cases covered