feat: implement reconciliation loop (#17261)

Closes https://github.com/coder/internal/issues/510

<details>
<summary> Refactoring Summary </summary>

### 1) `CalculateActions` Function

#### Issues Before Refactoring:

- Large function (~150 lines), making it difficult to read and maintain.
- The control flow is hard to follow due to complex conditional logic.
- The `ReconciliationActions` struct was partially initialized early,
then mutated in multiple places, making the flow error-prone.

Original source:  

fe60b569ad/coderd/prebuilds/state.go (L13-L167)

#### Improvements After Refactoring:

- Simplified and broken down into smaller, focused helper methods.
- The flow of the function is now more linear and easier to understand.
- Struct initialization is cleaner, avoiding partial and incremental
mutations.

Refactored function:  

eeb0407d78/coderd/prebuilds/state.go (L67-L84)

---

### 2) `ReconciliationActions` Struct

#### Issues Before Refactoring:

- The struct mixed both actionable decisions and diagnostic state, which
blurred its purpose.
- It was unclear which fields were necessary for reconciliation logic,
and which were purely for logging/observability.

#### Improvements After Refactoring:

- Split into two clear, purpose-specific structs:
- **`ReconciliationActions`** — defines the intended reconciliation
action.
- **`ReconciliationState`** — captures runtime state and metadata,
primarily for logging and diagnostics.

Original struct:  

fe60b569ad/coderd/prebuilds/reconcile.go (L29-L41)

</details>

---------

Signed-off-by: Danny Kopping <dannykopping@gmail.com>
Co-authored-by: Sas Swart <sas.swart.cdk@gmail.com>
Co-authored-by: Danny Kopping <dannykopping@gmail.com>
Co-authored-by: Dean Sheather <dean@deansheather.com>
Co-authored-by: Spike Curtis <spike@coder.com>
Co-authored-by: Danny Kopping <danny@coder.com>

coderd/util/slice/slice.go

@@ -90,6 +90,17 @@ func Find[T any](haystack []T, cond func(T) bool) (T, bool) {
 	return empty, false
 }
 
+// Filter returns all elements that satisfy the condition.
+func Filter[T any](haystack []T, cond func(T) bool) []T {
+	out := make([]T, 0, len(haystack))
+	for _, hay := range haystack {
+		if cond(hay) {
+			out = append(out, hay)
+		}
+	}
+	return out
+}
+
 // Overlap returns if the 2 sets have any overlap (element(s) in common)
 func Overlap[T comparable](a []T, b []T) bool {
 	return OverlapCompare(a, b, func(a, b T) bool {