Rewrite Sequential + Parallel for context flow

[santib]

Summary

• Sequential: #run wraps input in ExecutionContext, threads it through steps calling formatter.format_input → step.run → formatter.format_output → context.record, with before_run/after_run hooks at each step
• Parallel: #run forks context per branch (independent current_input, shared global_state), merges children back, passes merged context to aggregator
• ExecutionContext: #record now symbolizes step names for consistent ctx[:step_name] access
• Sequential.build DSL: declarative workflow construction via block (step MyStep)
• Specs fully rewritten with proper Runnable subclasses testing context flow, hooks, and formatters

Phase 5 of the v2 refactor. Based on #28.

Test plan

• Sequential threads context: step 2 reads step 1's output via context[:step1]
• Sequential hooks fire in order, formatters transform input/output
• Sequential accepts existing ExecutionContext (preserves global_state)
• Parallel branches get independent current_input
• Parallel branches share global_state
• Parallel merges child outputs into parent context
• Sequential.build constructs workflow from block
• Error propagation preserved in both workflows
• 70 specs pass, 0 failures
• Rubocop clean

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