feat(engine): workflow interpolation for ${input.*} and ${steps.*} (issue #16)

[leo-aa88]

Summary

Implements MVP workflow string interpolation from the design doc §13.1 (dot paths only, non–Turing-complete): ${input...} and ${steps...} against a small run context, with §13.2 step result shape (output, meta).

API (internal/engine/interpolation.go)

• StepResult: Output any, Meta map[string]any (maps hold nested fields for dot lookup).
• Context: Input map[string]any, Steps map[string]StepResult.
• InterpolateString(s, ctx): finds ${...} tokens (inner path trimmed), resolves paths, substitutes string values (scalars formatted; objects/arrays JSON-encoded).
• InterpolateWalk(v, ctx): recursively interpolates string leaves in map[string]any and []any (for workflow with trees).

Path rules

• input.<key>[.<nested>...] — walk Context.Input.
• steps.<step_id>.output[.<nested>...] or steps.<step_id>.meta.<key>... — walk StepResult.Output / Meta as map[string]any; drilling into non-maps returns a clear error.

Errors

• Unknown keys / unknown step: messages include undefined path or unknown step where applicable (validation-friendly for callers).
• Type mismatch when a segment needs a map but value is not: cannot resolve with %T.

Tests

• §7.4 / §13.1 style: repo=${input.repo}, number=${input.number}, whole ${steps.fetch_pr.output}, nested ${steps.review.output.summary}, ${steps.fetch_pr.meta.durationMs}.
• Unknown placeholder: ${input.unknown_key}.
• Unknown step, drill into non-map output, empty ${}, whitespace inside token, InterpolateWalk on nested maps.

Verification

• make fmt (go fmt ./...)
• make test (go test ./... -race)
• make vet

Closes #16

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