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>

codersdk/deployment.go

@@ -791,6 +791,19 @@ type NotificationsWebhookConfig struct {
 	Endpoint serpent.URL `json:"endpoint" typescript:",notnull"`
 }
 
+type PrebuildsConfig struct {
+	// ReconciliationInterval defines how often the workspace prebuilds state should be reconciled.
+	ReconciliationInterval serpent.Duration `json:"reconciliation_interval" typescript:",notnull"`
+
+	// ReconciliationBackoffInterval specifies the amount of time to increase the backoff interval
+	// when errors occur during reconciliation.
+	ReconciliationBackoffInterval serpent.Duration `json:"reconciliation_backoff_interval" typescript:",notnull"`
+
+	// ReconciliationBackoffLookback determines the time window to look back when calculating
+	// the number of failed prebuilds, which influences the backoff strategy.
+	ReconciliationBackoffLookback serpent.Duration `json:"reconciliation_backoff_lookback" typescript:",notnull"`
+}
+
 const (
 	annotationFormatDuration = "format_duration"
 	annotationEnterpriseKey  = "enterprise"