feat(coderd): add task log snapshot storage endpoint (#21644)

This change adds a POST /workspaceagents/me/tasks/{task}/log-snapshot
endpoint for agents to upload task conversation history during
workspace shutdown. This allows users to view task logs even when the
workspace is stopped.

The endpoint accepts agentapi format payloads (typically last 10
messages, max 64KB), wraps them in a format envelope, and upserts to the
task_snapshots table. Uses agent token auth and validates the task
belongs to the agent's workspace.

Closes coder/internal#1253

coderd/database/queries/tasks.sql

@@ -75,3 +75,22 @@ WHERE
 	id = @id::uuid
 	AND deleted_at IS NULL
 RETURNING *;
+
+-- name: UpsertTaskSnapshot :exec
+INSERT INTO
+	task_snapshots (task_id, log_snapshot, log_snapshot_created_at)
+VALUES
+	($1, $2, $3)
+ON CONFLICT
+	(task_id)
+DO UPDATE SET
+	log_snapshot = EXCLUDED.log_snapshot,
+	log_snapshot_created_at = EXCLUDED.log_snapshot_created_at;
+
+-- name: GetTaskSnapshot :one
+SELECT
+	*
+FROM
+	task_snapshots
+WHERE
+	task_id = $1;