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/prebuilds/api.go

@@ -0,0 +1,27 @@
+package prebuilds
+
+import (
+	"context"
+)
+
+// ReconciliationOrchestrator manages the lifecycle of prebuild reconciliation.
+// It runs a continuous loop to check and reconcile prebuild states, and can be stopped gracefully.
+type ReconciliationOrchestrator interface {
+	Reconciler
+
+	// RunLoop starts a continuous reconciliation loop that periodically calls ReconcileAll
+	// to ensure all prebuilds are in their desired states. The loop runs until the context
+	// is canceled or Stop is called.
+	RunLoop(ctx context.Context)
+
+	// Stop gracefully shuts down the orchestrator with the given cause.
+	// The cause is used for logging and error reporting.
+	Stop(ctx context.Context, cause error)
+}
+
+type Reconciler interface {
+	// ReconcileAll orchestrates the reconciliation of all prebuilds across all templates.
+	// It takes a global snapshot of the system state and then reconciles each preset
+	// in parallel, creating or deleting prebuilds as needed to reach their desired states.
+	ReconcileAll(ctx context.Context) error
+}