Refactor template::resolve() params into TemplateContext struct

## Context

PR #23 added the `execution.*` template namespace, bringing `resolve()` to 5 parameters. This will continue to grow as new namespaces are added.

## Suggestion

Group the parameters into a struct:

```rust
pub struct TemplateContext<'a> {
pub input: &'a HashMap,
pub config: &'a HashMap,
pub secrets: &'a HashMap,
pub execution: &'a HashMap,
}
```

Then `resolve()` becomes:

```rust
pub fn resolve(
value: &Value,
ctx: &TemplateContext,
) -> Result
```

## Scope

- Update `resolve()`, `resolve_string()`, `resolve_variable()` signatures
- Update all call sites in `pipeline.rs`
- Update all tests in `template.rs`
- Consider adding a builder or helper for `TemplateContext` construction