Add process and I/O execution foundation

[Mikhail Shirkov (shirkevich)]

Summary

This is PR 1 for the DAG concurrent execution rollout. It introduces the reusable process and stream-isolation foundation without enabling scheduler behavior or changing Terraform bulk routing.

Changes:

• Add pkg/process with Runner, TaskSpec, Streams, Result, default os/exec runner, context-aware execution, cancellation reporting, and exit-code preservation.
• Extend pkg/io with prefixed per-node stream composition for terminal, file, and capture sinks.
• Refactor internal/exec.ExecuteShellCommand() into a backward-compatible wrapper over pkg/process while preserving CI stdout/stderr capture options.
• Replace the runTerraformShow() global os.Stdout swap with injected stdout capture.

Scope

No scheduler, CLI routing consolidation, concurrency flags, or Terraform adapter behavior is enabled in this PR.

Validation

rtk env GOCACHE=/private/tmp/atmos-gocache GOMODCACHE=/private/tmp/atmos-gomodcache go test ./pkg/process ./pkg/io ./internal/exec ./cmd/terraform

Next PR

PR 2 should branch from codex/dag-process-io-foundation and add the generic pkg/scheduler core with ready-queue scheduling, bounded workers, deterministic aggregate results, and isolated unit tests only.

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[Erik Osterman (Cloud Posse) (osterman)]

looking forward to it Mikhail Shirkov (@shirkevich) !

[Mikhail Shirkov (shirkevich)]

Superseded by same-repo draft PR #2464, which uses cloudposse/atmos as the head repository and keeps this stack branch based on main.