feat(session): add automatic retention cleanup for archived sessions

Add configurable session retention that auto-deletes archived sessions
older than retention.days (default: 30, 0 = disabled). Runs every 6
hours via Scheduler, batched at 100 sessions per run.

Also call FileTime.remove() on session archive and delete to clean up
per-session read timestamps.

The database had grown to 1.99GB with 1,706 sessions spanning 53 days
because there was no automatic cleanup — sessions were only soft-archived
but never deleted.

Author: BYK

packages/opencode/src/config/config.ts

@@ -1194,6 +1194,16 @@ export namespace Config {
           url: z.string().optional().describe("Enterprise URL"),
         })
         .optional(),
+      retention: z
+        .object({
+          days: z
+            .number()
+            .int()
+            .min(0)
+            .optional()
+            .describe("Auto-delete archived sessions older than this many days (default: 90, 0 = disabled)"),
+        })
+        .optional(),
       compaction: z
         .object({
           auto: z.boolean().optional().describe("Enable automatic compaction when context is full (default: true)"),