[Parent] Compiler internals and pipeline generation improvements (IR types, YAML gen, prompt decoupling)

Groups open issues that improve the ado-aw compiler's internals, typed IR, and pipeline YAML generation quality.

| Issue | Area | Summary |
|-------|------|---------|
| #1081 | IR types | Add `EnvValue::RuntimeExpression` to prevent `$[]` from leaking into step `env:` blocks |
| #987 | Compile structure | Split canonical agentic-pipeline shape out of `standalone_ir.rs`; move Agent-job condition contributions into Declarations |
| #384 | YAML generation | Replace manual string building in `compile_gate_step_external` with typed IR |
| #432 | Runtime injection | Inject runtime parameter values into agent prompt body via ado-script (sanitization at runtime, not compile time) |
| #237 | Output decoupling | Decouple prompt body from compiled pipeline output (runtime-import approach) |

Potentially related issue noted but not linked (lower confidence — may already be implemented): #860 built-in PR context precomputation.

> Generated by [Issue Arborist](https://github.com/githubnext/ado-aw/actions/runs/28530704230) · 110.7 AIC · ⌖ 10.6 AIC · ⊞ 6K · [◷](https://github.com/search?q=repo%3Agithubnext%2Fado-aw+is%3Aissue+%22gh-aw-workflow-call-id%3A+githubnext%2Fado-aw%2Fissue-arborist%22&type=issues)

Treat this as a coordination issue rather than a standalone change. Read the listed child issues first, especially the compiler structure work around standalone_ir.rs and YAML generation entry point compile_gate_step_external. Done means selecting one scoped child issue with its own implementation and validation criteria; this parent is complete only when the listed improvements are addressed.

Written by the indexing model from the issue text.