feat: add typed workflow branching

[haseebahmed248]

Summary

Add explicit workflow decisions without moving control flow into host adapters. The compiler checks every final-result path, and selected branches share the runtime's existing permissions and budgets.

This is a draft pending the public RFC review in #51. Please record the RFC outcome before merging; this PR does not publish to npm.

Related issue

Closes #20.

RFC: #51. The deadline-dispatch guard also relates to #34.

Changes

• Add if/else, else-if, nested blocks and explicit fail statements.
• Check boolean conditions, lexical scopes, incompatible returns and unreachable/incomplete paths.
• Count the maximum calls per path, including continuations but excluding continuations after a terminating arm.
• Introduce IR v2 for control flow while preserving v1 output and generated contracts for linear sources.
• Execute selected blocks with shared counters, cost and deadlines; prevent dispatch at an exhausted deadline.
• Generate typed model contracts for each arm and add a keyless CLI example.
• Cover branch diagnostics through the compiler and CLI, and execute the example from the actual installed npm tarball.

Validation

Local verification: Node.js 25.2.1, npm 11.6.2. The existing CI matrix covers Node.js 22/24 and package checks on Linux, macOS and Windows.

• npm run check
• npm run check:generated
• npm test — 167 tests passed
• Additional validation is described below, or was not needed.

Additional validation:

• Clean dependency installation with npm ci.
• npm run test:release — 4 tests passed.
• npm run test:package — 115 package files, 7 public exports and both CLI aliases; generated v2 contracts type-check in an isolated consumer and both aliases pass all 5 branching scenarios.
• git diff --check.

Risk and compatibility

• Security or permission effects: branches do not grant authority or reset accounting. Only selected effects execute. Already-started model calls may still incur charges before cost enforcement. Host adapters and compiler IR remain trusted, not sandboxed.
• Compatibility or breaking changes: compileSource returns WorkflowIr (v1 | v2). Explicit v1 consumers must narrow the version. Old npm runtimes cannot consume v2. Nested model contracts use path-qualified names. The event schema remains v1.
• No union/optional narrowing, match, loops, retries or implicit branch-result joins are introduced. The supported subset and pending design questions are documented in the RFC and docs/BRANCHING.md.
• README installation work assigned in Make the npm alpha easy to find and install #30 and the illustrative-example issue Replace the README's illustrative example with current GuardStep syntax #49 are untouched.

Checklist

• This change is focused on one issue.
• Behavioral changes include tests.
• User-facing behavior and unsupported constructs are documented.
• Generated files are current.
• No credentials, private data, or unredacted model output are included.
• Commits include the required DCO Signed-off-by line.