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:  

https://github.com/coder/coder/blob/fe60b569ad754245e28bac71e0ef3c83536631bb/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:  

https://github.com/coder/coder/blob/eeb0407d783cdda71ec2418c113f325542c47b1c/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:  

https://github.com/coder/coder/blob/fe60b569ad754245e28bac71e0ef3c83536631bb/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 {

coderd/util/slice/slice_test.go

@@ -2,6 +2,7 @@ package slice_test
 
 import (
 	"math/rand"
+	"strings"
 	"testing"
 
 	"github.com/google/uuid"
@@ -82,6 +83,64 @@ func TestContains(t *testing.T) {
 	)
 }
 
+func TestFilter(t *testing.T) {
+	t.Parallel()
+
+	type testCase[T any] struct {
+		haystack []T
+		cond     func(T) bool
+		expected []T
+	}
+
+	{
+		testCases := []*testCase[int]{
+			{
+				haystack: []int{1, 2, 3, 4, 5},
+				cond: func(num int) bool {
+					return num%2 == 1
+				},
+				expected: []int{1, 3, 5},
+			},
+			{
+				haystack: []int{1, 2, 3, 4, 5},
+				cond: func(num int) bool {
+					return num%2 == 0
+				},
+				expected: []int{2, 4},
+			},
+		}
+
+		for _, tc := range testCases {
+			actual := slice.Filter(tc.haystack, tc.cond)
+			require.Equal(t, tc.expected, actual)
+		}
+	}
+
+	{
+		testCases := []*testCase[string]{
+			{
+				haystack: []string{"hello", "hi", "bye"},
+				cond: func(str string) bool {
+					return strings.HasPrefix(str, "h")
+				},
+				expected: []string{"hello", "hi"},
+			},
+			{
+				haystack: []string{"hello", "hi", "bye"},
+				cond: func(str string) bool {
+					return strings.HasPrefix(str, "b")
+				},
+				expected: []string{"bye"},
+			},
+		}
+
+		for _, tc := range testCases {
+			actual := slice.Filter(tc.haystack, tc.cond)
+			require.Equal(t, tc.expected, actual)
+		}
+	}
+}
+
 func TestOverlap(t *testing.T) {
 	t.Parallel()