feat(step.foreach): add item_var, single-step, and foreach.index support

[Copilot]

• Explore codebase to understand existing step.foreach implementation
• Add item_var config key support (item_var is now canonical; item_key retained for backward compat)
• Add step (singular) config key support as alternative to steps
• Add {{.foreach.index}} access via a foreach map injected into child context
• Update cmd/wfctl/type_registry.go ConfigKeys for step.foreach
• Fix: thread app parameter from StepFactory to sub-step factories (was using captured outer app=nil)
• Fix: remove outer app param from NewForEachStepFactory (matches NewRetryWithBackoffStepFactory signature)
• Fix: validate step/steps types explicitly; error when both are provided
• Fix: replace duplicate foreachBuildSubStep with shared buildSubStep helper
• Fix: guard foreach map injection to avoid overwriting conflicting user keys
• Add tests: mutual exclusivity of step/steps, wrong types, app threading, conflict guard
• All 18 foreach tests pass, full build clean

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[Copilot]

Pull request overview

Adds missing spec features to the step.foreach pipeline step so foreach iterations can bind a custom item variable name, define a single sub-step via shorthand, and reference an iteration-scoped foreach map in templates.

Changes:

• Add item_var as the canonical per-item binding key (keeping item_key for backward compatibility).
• Support step (singular) as an alternative to steps for single sub-step configurations.
• Inject foreach context (including foreach.index) into each iteration; update wfctl type registry and add tests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
module/pipeline_step_foreach.go	Adds item_var, step shorthand parsing, and injects foreach map into child context.
module/pipeline_step_foreach_test.go	Adds tests for item_var, step shorthand, foreach.index, and error cases.
cmd/wfctl/type_registry.go	Updates step.foreach ConfigKeys to include new supported options.

[intel352]

@copilot apply changes based on the comments in this thread